Jim, not to discount Ricks observation which is the most likely, but it might

also be a possibility that there is no vapor barrier and moisture vapor is

collecting on the back of the tile and causing it to blister.

There is the moisture line on the side will that looks like a past flood, but excessive moisture can also wick up onto walls.

Bottom line, there is water damage. Report what you see since you don't

have a crystal ball or time machine to see what happened in the past.

__________________
Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002