Xp deducted unexpectedly

Question

Today I got to know that my 27XP has been deducted without losing any challange. My score shows -27 for today. What could be the reasons ?

Answer 1

A reason can be the removal of best-answer mark.

Answer 2

Or removal of thread holding best answer mark, or downvote/clear upvotes on best answer mark ^^

Answer 3

Maybe you have a best answer with 17 upvotes unmarked