I can only assume it must be poor writing/characterisation, because he's always seemed one of the more stable members of the Bat-clan before this point.
The guy lost almost every single person who has meant something to him in the within the past two years. The fact thatsome of them actually came back actually makes it worse because he had almost acheived a measure of closure over their deaths. It's a testament to his former stability that he hasn't become a raving pschopath by now. As it is, Tim is probably the most emotionally traumatized of the Batclan right now.