I think I read in the VPI book one legit reason why people cut open the eggs. Some do it early out of wanting to see what they got in the egg. But for those that wait until the slit's were in the egg from the snake, was to create an air bubble for the snake to retreat into to avoid drowning while they absorb their yolk.

It made sense to me, but people still do lose snakes in eggs that go to full term, and it could be either drowning ( a good reason to cut a window), or cutting and getting a vein because it was too early... or the snake popping out of the egg before he absorbs his yolk because there was a door open....