In general, this is the recent trend, with the tipping point somewhere between 1996 and the year 2000. Some of this transition reflects the ongoing process of Democrats and Republicans trading places as the liberal and conservative parties respectively, called realignment, that started around 1965 and has now largely run its course. See, for example, these charts:
Exit polls from 2020 reveal that the trends seen in 2016 and 2018 continued into the 2020 election.
If yes, is this true in general, or only for certain disciplines?
The class of people with higher education is heterogenous politically in a manner heavily influenced by industry and occupation as illustrated by the chart below using data from 2000 through 2012 but which probably remains true on a relative basis even though there is an overall shift to the left among more educated Americans:
Some fields like academics, entertainment, reporters, and tech company employees are consistently left leaning. Some fields like fossil fuels, agriculture, and construction are consistently right leaning.
No professions or industries are consistently moderate, but lawyers, pharma, real estate, finance and non-fossil fuel mining are bimodal.
In the case of lawyers, this largely represents alignments with core client bases. Personal injury and criminal defense lawyers lean left, insurance defense and corporate lawyers lean right. Professional political workers are likewise bimodal.
In pharma and non-fossil fuel mining, this is largely a reflection of left leaning scientific and technical workers v. right leaning corporate side executives.
In the case of real estate and finance this is largely a reflection of geography. Real estate and finance officials in large left leaning cities lean left, those in smaller towns and rural areas lean right.
Among people in business, middle to upper management and professional employees in big business (who aren't actually top management) tend to be to the left of owners and top level managers of small businesses.
Scientists, in general, lean left:
Among clergy, political leanings track denominations which very widely in their overall political tendencies. For example, mainline religious denomination clergy tend to be well to the left of clergy in predominantly white Evangelical Christian denominations.
Trends associated with gender, age, nativity (i.e. foreign born or not), race, ethnicity, religion, and geography, however, are comparable in magnitude to education and occupation.
Are there any studies investigating the causes?
Yes. But the answer to this subquestion is too long for me to provide with proper citations right now, and it can't be reduced to a pithy sentence or two without capturing some complex reasons related to history and intersectionality in addition to a few general trend rules and shifts in political party coalitions.
Generally speaking affluence and great economic security are associated with liberalism and secularism, while economic scarcity and economic insecurity are associated with conservatism and placing a high importance on religion in one's life, on a broad, cross-cultural basis. This is shown, for example, by the World Values Survey.
One starts from that foundation and then explores why exceptions have emerged in the U.S. historically from this general trend (e.g. why African-Americans in the U.S. have tended to be both politically liberal and religious relative to the general public, despite comparatively high levels of economic scarcity relative to the U.S. population as a whole, and why the white working class in the U.S. was once much more liberal than it is now when it has reverted to the cross-cultural long term norm).
In a nutshell, African-Americans (and non-Christians who would otherwise be inclined to be conservative) have leaned left because existential survival issues at the fate of conservatives who have demonized them has been more important for them.
American working class whites veered left from the end of World War II through the early 1970s, due to unprecedented prosperity and economic security, and a strong union movement, in this time period that steadily deteriorated with offshoring and automation, and with the post-World War II recovery of markets that the U.S. which was spared most of the devastation of World War II supplied upon they could meet their own needs again creating a thriving demand for exports in this time period. This situation has kept getting worse and come to a breaking point that has particularly pushed white working class men in the direction of extreme conservatism and has also mostly stripped them of their links to the union movement.
The rise of public sector unionization has also impacted the shift of educated people to the left. Unionization has thrived in the public sector, which has also grown steadily in share of employment, because it hasn't been subject to the same intense offshoring and automation pressures, and the shrinking of export markets, that the private sector has seen, and tends to require well educated employees.