Sean Marshall is striking out more than 12 batters per 9 IP, walking a hair over 2 and allowing 1 homer. Combined, this figures suggest an ERA exactly in line with the 2.26 he put up last year.
Unfortunately, his ERA is 4.24. Maybe he's just making a lot mistakes? Well, his GB/FB rate is up and his LD% is down. What gives? A .435 BABIP.
There's an article on Baseball Prospectus right now that looks at a very similar reliever from a results standpoint who's going through the same problem, Johnny Venters. He's got a .449 BABIP that is a big reason why he looks like he's scuffling despite pitching pretty well.
The article actually looks at his hits allowed and judges them based on luck and finds, not surprisingly, that he suffered from a rash of bloops and seeing-eye singles.
We can't automatically assume the same for Marshall, but I'd be willing to bet on a similar finding. If somebody had the time & energy, it would certainly be interesting to take a look.