It could be just coincidence or they might not like the idea of paying for your vacation. Personally, I don't charge for my vacation but do have a bit of a higher daily rate to compensate for that (although, I'm seeing more and more charge higher than me, which I think is good!)

I agree with 3rdtimesacharm though. If it's happening often than you may want to reconsider how you handle your holidays. Perhaps a daily fee increase might be the better option?