Recently, I heard that U has risen, and the bid-ask spread is quite high. Some people say that the bull market is returning, so many are buying U to catch the bottom.

So retail investors' psychology is based on their own emotions, or they might be influenced by market sentiment. When it rises, they say the bull is returning, so they buy U to catch the bottom → so U rises.
They fall into a kind of cause-and-effect reversal, believing their reasoning is logical.

Then I turn it around and ask, is there a possibility that they are not buying the bottom, but rather because they are about to be liquidated and need to add funds to cover margin? This is also a kind of reasoning; in any case, one must have their own judgment.