This answer is not meant to be definitive proof that most software developers are left leaning, but I do want to show some high correlations between your average software developer and your typical American democrat.
First, the average software developer is a young person. In the 2016 Stack Overflow developer survey (which isn't perfect, but is again this answer isn't going for perfect) Over 50% of software developers are under 30 years old. A 2016 from the Harvard institute of Politics show that Clinton was polling at 61% of people under 30. The NYT exit polls also show that Clinton won people under 30 by around a 20 point lead.
Second, most software development centers are in big cities. Silicon valley, NYC, San Francisco, Seattle, Austin, Houston, etc. These are all large urban areas that reliably vote blue.
Finally, and this is entering into total personal speculation space, I think the paradigms of software engineering tend be similar to a liberal point of view. Specifically, developers like to centralize; why write the same code twice? Why have the same data in 2 places? etc. Conservatives tend to be more about personal freedom, states right, and reduction of a large centralized government. (Note: I'm not taking a stance on which one is right or wrong here, please don't get into a war in comments).
Remember though that all I've really shown here is high correlation, not causation. That being said, when you roll the dice I think this will show why your average software developer is likely to be a liberal.