I have a theory, that when you feel under attack - mentally or physically - you start behaving the age you were during the most traumatic moment in your memory. This is backed up by my behaviour - truculent teen - and my hubby's - unreasonable toddler.
When I feel threatened, I get paranoid and grumpy and start acting like the selfish teenager I was. I think this is because I had some "issues" as a teen that stand out most in my mind compared to other happier memories. Hubby lost his Nana when he was 6 and it hit him hard, as he was very close to his Nana, so when he feels threatened he turns into a stubborn toddler.
This causes some interesting arguments in the Ratcliffe household, when Grumpy Teen meets Stubborn Toddler. Things get thrown, doors get slammed, feet get stamped. We both go off and sulk for a bit, then when we've both grown up again we get back together and sort it out like adults.