Bleeding hosts are a common Eucharistic "miracle," and no doubt the actual cause is similar in pretty much all cases (there are other instances where the cause was determined to be bacteria).
It's funny. The transformation of the host into the body and blood of Christ is supposed to be a miracle in itself; why the need for a further miracle on top of a miracle?
Also, the transformation is supposed to take place without changing the accidental properties of the hoat--in other words, it's Christ's flesh but still physically indistinguishable from a wafer. (Which basically makes transubstabtiation unfalsifiable, but I digress.) Why would something that is supposed to be, by all appearances, bread, bleed? What organs is it bleeding from?
Eucharistic miracles are generally pretty low-budget, anyway, not unlike the gold dust and angel feathers that pass for miracles in charismatic churches.