In the last two seasons, Mark Teixeira of the Yankees saw his batting average drop while still hitting for power. During the the 2008-2009 seasons he posted a .300 BA with a .259 isolated power (Extra bases per at bat). Since the start of the 2010 season, Mark hit .253 with a .245 isolated power. He's still producing extra base hits, but his singles have fallen.
It appears that Mark changed his approach at the plate in order to hit more home runs. In the 2008-2009 period, Teixiera popped up 8.4% of balls in play about compared to 11.6% in 2010-2011. That doesn't seem like much, but in that last season and a half, he popped up as much as in 2008-2009, 83 times. He has two hits on those 166 pops, all in the early two seasons.
The worrisome fact, however, is that Mark is popping up a different set of pitches.
Note the pitch movement. Mark was swinging at pitches that didn't drop as much as expected (pitches above the X axis. On top of that, these were fairly high in the strike zone. In other words, a player with a level swing might be fooled by a "rising" pitch high in the zone and get under it a bit, popping it up.
In the last season, Teixeira continues to pop up pitches with the same movement, dropping less than expected. What's changed in the location of most of those pitches, now low in the strike zone. This looks like someone trying to uppercut the ball in order to drive it out of the park. In this case, Mark is popping up pitches, because he's swinging lower. A level swing hit those square, resulting in a line drive.
It seems that Teixeira tries to square up the ball less and hit it out of the park more. With him tied for the AL lead in home runs and third in RBI, it seems to be a good trade off for the Yankees. The change did drop his OBP over 30 points, so all those home runs and runs are coming at a higher price in outs. If pitchers can exploit his tendency to pop up, that trade off could look a lot worse.