I read an article about it years ago. According to it, the Allies were supposed to dump those weapons into the ocean, but they didn't care to go so far, so they dropped them into the Baltic instead.
1. Hang-ups, where the release command was given over target but the ordnance didn't separate. Usually this requires the entire weapon rack ( such as a BDU ) to be ejected using the emergency explosive charges.
2. Trap-weight limits; the old-gen F/A-18s were crippled by this problem, wherein they couldn't recover to ship carrying the ordnance with which they launched. Particularly if they topped-up from a tanker on the return leg this could necessitate dumping unused ordnance.
I think #2 is much less of a problem now with the Super Hornets.
In general, after several decades of toting 2,000 lb Mk 84 bombs, ordnance is returning to smaller sizes due to the concentration of effect possible with precision guidance. 500 lb bombs are commonplace now, and sometimes even smaller. So this should also reduce the frequency of dumping.
http://news.discovery.com/earth/oceans/u-s-accidentally-bomb...