There are rumours for quite some time, some journalists were talking about it a few weeks ago, that Haas wasn't happy with Mick because he, apparently, isn't a teamplayer. It apparently have something to do with the sprint race in austria. I don't exactly remember what happened here but the team (i don't know if it was steiner or Gene haas) didn't like the way he behave. I wouldn't be surprised if this story is totally made up by haas to find a excuse to fire him.
I agree that he crashed too much at the beginning of the year and that haas can't afford it but he has improve a lot since silverstone i would say and the goals they set for him to keep his seat were just unreachable. It is all really unfair. And i am not even talking of all the times they messed up his strategy.
And you can't pretend to be an ambitious team by choosing hulkenberg as a driver. Nothing against the guy but if you are ambitious then bet on a young, promising guy who can progress not someone who has done 3 races in 2 years and has never been seen as a top driver.