even the OED is uncertain on this, but suggests perhaps alluding to the crow's traditional association with violent death, or, as suggested in quot. 1939, to its harsh and raucous cry.

on a related note..
Four scholars at Oxford were making their way down the street, and happened to see a group of ladies of the evening. “What’s this?” said the first. “A jam of tarts?” “Nay,” said the second, “an essay of Trollope’s.” “Rather, a flourish of strumpets,” advanced the third. “No, gentlemen,” concluded the last. “Here we have an anthology of pros.” [thanx to Quinion]