The past week the invalid rate has been rising a bit, because of the increased network speed. Invalid shares occur each time right after a block has been reported, normally every 10 minutes. They even out to a certain percentage of your hashrate. Since blocks were reported more often recently, invalids have been at a higher percentage too.

We've also been playing around with server improvements that required some restarts, which may have also slightly added to your invalid rates.

Restart your miners to take full advantage of the improvements!