Although snakes can randomly do this, two snakes doing it several times suggests an underlying problem. I have the same question as Mustang5 - are you feeding F/T? I suspect that you probably are, and I suspect that the prey isn't completely thawed/warmed. That is typically the cause. The outside of the rodent can seem warm, but then center may still be cold, and this can cause the snake to reject the prey. The easiest way to check this is, after you have warmed the prey the way that you normally do, drop it into a ziplock bag, then pinch the rodent between your fingers and hold them there for a while, at least 15-20 seconds, and if the rodent isn't ready, it will begin to feel cool/cold. Do this test both in the center of the torso and on the head. If you feel cool/cold, continue to warm the prey and repeat until the prey feels warm between your fingers for a good 15-20 seconds.

If you are feeding live prey, I don't know what to tell you other than maybe the prey are being raised on a snake-unfriendly substrate.