One possible reason could be that he's under a great deal of stress. Maybe there are new threats to Berk that he's trying to deal with alone and it's getting to him. Another reason might be personal issues, like if he's having problems with his relationship with Astrid or his father. Or perhaps he's frustrated with the dragons not following his commands as he expects, which makes him lash out at everyone around him.
It might be that he's been influenced by some external force. In fanfiction, there could be a magic spell or a new type of dragon that has an effect on his mood. Or, he could be dealing with self - doubt. He's always been seen as a leader, but if he starts to question his abilities, it can make him irritable and cause him to snap at others. For example, if a new dragon - riding technique fails miserably, he may take out his frustration on those around him.