Re-asking a professor if I can write my thesis after I chose another one
All you should admit is "a discrepancy, strictly on a personal level, not subject-related". That must be sufficient. You might add you made an error in your first judgment (probably due to the limited time-frame for decision) - which actually seems to be the case: so don't lay the blame on prof. A.

Even if prof. B knows everything about the conduct/character or prof. A (which is more than likely!), he or she will probably appreciate integrity - coexisting with your ability and willingness to admit a mistake and to revise your decisions in order to find a solution.

Please, don't get prof. B into a situation where he has to (or could feel obliged to) comment on the actions of prof. A. They have to get along on a professional level until retirement (while students come and go, in a manner of speaking).