During the presidency of Bush his approval rating reached a high of around 85 % as he was close to 800 days in the office before beginning a continous downward slide that put him as low as 27,6 % by day 1308 of his presidency. While I would like to understand the general context that lead to this massive loss of approval over 500 days, I would like to focus on one timeframe in particular: At day 1007 his rating stood at 64,8 %, by day 1128 he was at 37,7 %, losing almost 30% in the span of 100 days. What was the cause?
GHW Bush ran on a platform that literally promised no new taxes. He famously said in his 1988 acceptance speech
Read my lips: "No new taxes!"
The problem was in 1990 Bush wanted to break a budget deadlock. The only way to do that was to compromise with Democrats and raise taxes
In September 1990, two years into George H.W. Bush’s first term, the United States was saddled with a $200 billion budget deficit. Months of partisan wrangling over possible tax increases and spending cuts had ended in a stalemate.
Despite the potential political backlash, Bush announced a compromise with congressional Democrats that would cut $500 billion from the deficit in five years, in part by raising “luxury taxes” on items including yachts and pricey cars, among other tax hikes.
It is still widely regarded as the reason why he lost the 1992 election
There is a combination of factors here, rather than one single reason. First off, HW Bush's popularity reached its height during the Gulf War, which had reasonable bipartisan support at the time, but his unwillingness to pursue Hussein back into Iraq and bring about regime change (as he had done previously in Nicaragua) was seen by many conservatives as a policy failure. This was aggravated by the fact that the US economy had started to slip prior to the gulf war, and Americans were increasingly worried that a war in the mideast would bring bring spikes in gas prices or actual shortages (as they had seen back in the Carter administration). He was forced to violate his own campaign promise — read my lips, no new taxes — because of a growing budget deficit, and had to face off against a conservative-leaning independent (Ross Perot) which produced some unpleasant optics and sowed dissent in conservative ranks.
All in all, HW Bush was stronger on foreign policy than domestic policy — natural, given his experience as ambassador to the UN and director of the CIA — but the foreign policy stumble with Iraq combined with worsening economic conditions domestically created a public opinion downturn. HW's public approval rate did recover, but only in the lame duck period after the election went to Clinton.
If I don't misread the timeline, this would be leading up to and right after, his losing the elections against Clinton. During the period running up to the election, he would have been under massive pressure. I suspect attack ads, political debates with Clinton - it's the economy, stupid, and general gaffes - there was something about marveling at supermarket scanners which made it seem he never shopped himself, for example.
Might also be that the shine came off Gulf War 1, where everyone was happy at a job well done, but then gradually got disappointed that Saddam hung in and that the Shia revolts were left out to dry.
None of these would yield an aha moment and Bush may not even be an outlier as a retrospective of polling losses nearing election time for a politician who went on to lose an election.