It's always really fluctuant throughout WoW during this time of the year. The school year is ending which keeps many people busy, it's near the time for another patch, so some people have hit road blocks in the current raid set or maybe aren't finding it as fun now because they've been doing this set for a while, and it's almost summer, so some people are going on vacation.
Although some servers may be hurt because of these problems, not all server will feel it.