I tend to have a hard time in summer for much the same reason I have a hard time over the Christmas holidays. What's vacation time for most is time when I'm trying to maintain my schedule while also trying to play ball, at least a little, with family members or friends who are doing whatever. There's usually a lot of drama, too.
From a writing standpoint, it can be hot or unpredictable weather-wise, and the readership/discussion tends to dip a little, or at least stall, with people on vacation, or just enough people that some of the usual conversation-starters aren't around. I know I often miss the guy who comments and points people to Topwebfiction after most chapters.