Would that be the sign of a guy w/ an advanced understanding of the strike zone? Nobody on, the opposing pitcher might nible a bit, get Alonso to swing at a pitch or 2 out of the zone putting him in more pitchers counts?. Runners on, and the oppossing pitcher has to be aggressive and Alonso sees more pitches he's capable of hitting, and takes advantage of those hitable pitchers? Perhaps he has solid bat control and can take advantage of the holes in the infield created by runners on 1st base and whatnot????
Or perhaps he's just clutch