I had an issue yesterday when I tried to cook some root vegetables in my sous vide supreme. I chopped them to roughly 1" lengths and placed them in a few quart Ziploc freezer bags and vacuumed them using a ThriftyVac. After just a few minutes in my sous vide at 84C the bags expanded with a lot of air and were very buoyant. I have not had this issue when sealing beef or fish, and no liquid seeped in or out of the bag, only air. The bags were very well sealed. It happened with my bag of parsnips, carrots, and bag of potatoes, but it did not happen with my beets.
Believe it or not, veggies just happen to contain a lot of air. Even under vacuum this is a fairly common occurrence when cooking veggies low temp. You can add oil or other liquid to the bag, it will help, but air pockets will still likely develop. I usually add weights to the bag or bleed the air out by lifting a corner of the bag and resealing by displacing the air as it happens.
I use ordinary Ziploc bags opened, clipped to the side of my cooking vessel. When placing vegetables in bags there are many pockets of air between them making the bags rather buoyant. My solution is to take a pair of long tongs with a locking clamp mechanism and secure the air-filleed bag below the water level. The water does displace most of the residual air from the bags but the ambient temperature of the bath still cooks the vegetables to the desired degree of doneness.