I think the reason we were so poor defensively at the start of the season is because pre-season, no work was done on the defence. We were looking for a Carsley replacement, winger and striker.
Last season, the defence was so awesome that no-one was bougbt, sold or shuffled in defence over the summer, and I think the expectation was that they would just carry on this season.
Because of the lack of attention, they started crap. Moyes shifted his attention back to them, they improve.
I voted "collectively" as no one man makes a bad defence.