How close were you to the exploding ship? It could possible have flung a funnel or cranemast or whatever in the air and dumped it on your head. Though luck there .
The mission/campaign editor doesn't show any mines in BF17 during the whole war. So that can't be it. It doesn't seem to be aircover either. The airbases don't reach that far in 1939. (but almost) Plenty of convoys and merchants run there though. So it could also have been that you were too busy looking at your prey going down and some escort of an approaching convoy which came to help got in reach to put some shells on your head. But your hydrophone operator should have picked it up as it approached.