Mostly abdominal pressure. The real problem is a stoma prolapsing out, not sucking in. Same reason that when you get a hernia things poke out making a bulge not sucking in making a dent.
Immediately after stoma creation and particularly in obese patients perioperative swelling can cause a stoma to pull back into the abdomen (which is a disaster) but after a few days the swelling goes down and prolapse become the risk.
To envision a stoma prolapse imagine holding the edges of the open end of a sock and then turning it partially inside out by pushing the toe inside towards the opening.
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