The stock market (especially tech) tanked, other investors started having cold feet, and he realized his purchase was a mistake.
It is bizarre though that Twitter leadership/board continued to engage with him on the matter – even handing him internal data to analyze – expecting a good faith resolution. Nothing Musk has done in the last few months has been in good faith. You either lawyer up and force him to stick to the agreement, or take the loss and move on. Appeasement isn't going to work.