This is a difficult question, one that has bothered me for years. When I ask professors to explain it, they do so with only one-sentence, circular reasoning, like, "It bleeds because its ischemic, duh."
Logically, how it is possible that LACK of blood flow (aka ischemia) can CAUSE bleeding?
I can only conjecture because I've never gotten a logical, full explanation from any source.
But I suppose it might have to do with a reperfusion injury. For example, we know that in the kidney or brain or lung or any other solid organ, a hemorrhagic lesion causes a pooling of blood that can settle in any dependent area, while in contrast, ischemia will cause a wedge. The wedge represents the area once nourished by blood flow from the now-occluded artery. This wedge shape however, does not always remain pale. Sometimes, the tissue remains anoxic and dies in place, leaving a white infarcted wedge. Other times, collateral circulation reperfuses the area, making it red and bloody.
Perhaps the GIT, being well vascularized, reperfuses the ischemic area, making it bloody? Anyway, the GIT as we know sloughs constantly, so the anoxic tissue will get sloughed, rather than remain in place. Perhaps that creates a bleeding ulcer.