it's just that the most recent events are the ones that most affect us.  You've got some fraction - say a third - of the people who are resolutely on one side of things, another third who are resolutely on the other, and the remainder floating around somewhere in between.  The first two groups aren't going to waver much - maybe a little between Agree and Strongly Agree, but otherwise pretty much stationary - but the third group isn't particularly happy on either side.  The most honest answer most of the time is the one the pollsters most hate - and take pains to avoid - which is "I don't know or care."  If they're pushable (and since they're not committed to either side, I think this group's most likely to be so), they'll slide away from center - and the event that they're most aware of is whatever just hit the newspaper.  

Probably needless to add: this is all uninformed speculation.  Ask me tomorrow and I'm sure I'll have a different answer. ;-)