I don't think it was a breakdown as such, tiredness and stress from covering other people's patients built up and then failing to correctly diagnose the brittle bone baby made him think he was going to get shell shock again.
Plus, cake always cheers me up! 
They've been building up to Mrs T becoming nurse shelagh all series, lots of comments about how much she misses it. And with all the PC-ness going on, it seems inevitable she'll become a working mum.