Why did my diff die?

[Captain nemo]

Cougar SV, only done 6 heats with it, yesterday it started to run slow, toward the end of the heat it was hardly moving, when i stripped it back the diff was fubar.

the seats for the ball race had worn right through the gear and almost cut it in half, in fact when i prized the bearing out of the centre of it, it fell into 2 pieces.

im assuming that the diff either came lose or i didnt tighten it enough originally, id guess the latter, it must have been loose enough for the balls to come out of their seats and wear away the gear,

everything else in the diff was fine, with exception that the circlip could turn
when i took the diff apart the grub screw and the screw through the outdrive didnt seem loose?

so was this my own dumb fault when i built it originally ?