Yes it's 5 weeks into the season, but it's also his 3rd season, so they know him pretty well. They said they didn't think he was communicating well with this year's staff.
One of the beat reporters-Brian McPherson, I believe-kept a tally on Nieves's mound visits this year. His numbers showed that after 19 visits, the next batter had a hit or walk 15 times. Obviously that in itself didn't get him fired, but you could look at it as symptomatic of a problem.
The bottom line is, none of us know jackshit about why he really got fired. We probably never will.