I don't know for sure, but I do know both my children had/have multiple movements a day at that age. My son's 23 months now, and he just got over a couple of weeks of that--he's back to 1-2 now. I always wonder if it's teething. They both drooled/drool a lot when they're teething, so I thought maybe that sped things along :-)
My daughter, now five, started her current once a day schedule when she was about three.
My friends always wondered why I complained so much about changing diapers and the cost of diapers until I told them it was three or four times a day!
Hope that helps :-)