Sounds like you vented a cell. Either from overheating or a bad cell.

Some Big NIMH packs don't like aggressive fast charges. I used to consider a 30 minute charge aggressive for my small cells. A lot of larger cells tend to consider a 4 hour charge as a "fast" charge.
Nicds on the other hand can handle a more aggressive charge (15-30 minutes usually), though slower charges heat them up less and result in longer lives.

D.