Originally Posted by al40
What I didn't do was completely fill the cavity before pushing the oil seal on (various forums recommended not to do this with scare-mongering of bearings overheating with too much grease),
As far as I am aware, the more grease you get in (and conversely, the less air you leave in) the better. Not sure a) what "too much grease" would mean, nor b) how that would cause a bearing to overheat.
Did the grease appear to be milky or creamy? If so, water got in somewhere (usually through the grease seal, though the dust cover/bearing buddy is a possibility as well.) Salt water, even when mixed in with grease, will do a number on the bearings in no time.
If not and the grease appeared normal, I'd suspect some contamination (sand, dirt, metal shavings, etc.) got in with the grease (though I'd also suspect that it had to be a lot of crap to take it out that quickly.)
One thing to be careful about is the use of bearing savers: Pumping grease in is good, but pumping in too much may invert the grease seals on the inside and allow water ingress. You should pump the grease in until the spring loaded flange *almost* hits its stop. Don't go nuts and pump grease in 'til it oozes out somewhere (unless you have spindle-lube type hubs.)