It seemed to happen a bit with Carlos Lee last year too.
Pre All-Star -- Slugged .528 (22 HR, .864 OPS)
Post All-Star -- Slugged .437 (10 HR, .747 OPS)
With Carlos, that's a little more reasonable, largely because he's such a streaky hitter. Abreu really took a dive though, which was so unusual seeing as how he's quite consistent.