It could be from leaving them in the water too long, or the water is too hot. Every once and awhile one may pop but it should not be a regular thing.

A rat/mouse has a living body temp just over 100F. I thaw my rats in warm tap water for about 45-60 mins. These are S, M, XXL. After they are all thawed I refresh the water with water just over 100F to get them into the correct range.

I used to use freezer bags and try to keep the rats dry but the bags almost always leak so now they go straight into the water. I put all the feeder in small plastic tub and thaw them in it in the bathtub. Then when I refresh the water I carry the entire tub into the snake room so they stay warmer longer. I then offer them without drying them at all. Stuck substrate has never been an issue.