That happened to me when i tried to fill the pot with 7.5-8 gallons of water. I use a ten gallon pot and a few bigger grain bills did wick the wort up my viole bag and outside the pot. That was with 20+ lb grainbills. I now always use 7 gallons and havent had the problem again. The most grain i have used with 7 gallons has been in the 10-16 range so i might use less water for my high gravity brews in the future.