Withers in 14w11b are neutral, like wolves. They have a passive behavior until hit, in which they attack the player like normal. This should not happen as withers are always hostile. Bug does not happen in 1.7.9, have not tested other snapshots yet.
*screenshots are below*
Duplicate of MC-29274 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.