I remember my #1 having the same problem at about that age. I actually sat in and observed her to see what was going on and I realised that at about 20-30min when she was coming out of deep sleep into light sleep, she was jerking. And the muscles jerks woke her up. I ended up swaddling her for naps again and she went back to 1.5hrs nap time. Suggest you could observe your child sleeping to see if it is the same thing?
Another thing is if you are putting them down for nap when they are over-tired, same thing can happen too. Also, I think there's a mental leap about then, and that means poor naps for some kids