Burner accounts are very common in sports, especially among players and families. Bryan Keller could have meant to tweet these from a burner account with 0 followers but accidentally did this from his main account.
While Keller could have been "hacked", he is not exactly high on any hackers list and if somebody wanted to hack a Keller, they would have hacked Clayton. It appears this is a sorry excuse for a mistake and if he were to just apologize outright and say he was upset, there would likely be much less drama.