I don't know that anyone "knows" what happened, to be honest.
I mean, many of us believe that they were in a relationship, but to be honest they never actually confirmed that.
As far as their "break up", again until they confirm it, no one will know for sure, but it doesn't have to be that they had some huge argument. It could have been something as simple as the pressures of maintaining a long distance relationship when you're young. Bash was in London preparing for a play and Joe was in Atlanta shooting Agatha, so that could have played some part in it, realizing that as their careers progress that they will probably spend more and more time apart.
Also, they are very young, as as much as I ship them, it probably makes sense to sow your wild oats as they say now, you know, when people really want your oats lol