Only occasionally, when freediving, I've done surface dives in which I have felt like water was being forced into my ear like an ear plug. I've had to abort the dive and I've even been "locked out" of diving for the day and unable to get water out of my ears after that occured. Despite being a trained swimmer and knowing several tricks to drain the ears, I was done. When it has happened, it has been during normal surface dives (I prefer one-legged dives), but with a bit more starting speed as I piked.
With this in mind, it could be the way the water is flowing into your ear canal in that position. I'm sure that individual ear shapes and contours can have some impact on how water travels past the canals and perhaps a "plugging" effect occurs in some people?
Another thought is that different people prefer different equalization techniques depending upon how their tissues and structures respond to pressure. Perhaps in that position, minute changes in your tissues or the pressure in your eustachian tubes in relation to your sinuses and mouth decrease your ability to equalize easily. Some divers have trouble clearing their ears when they first begin to dive and it could be possible that in such a position it is "new" to your body.
I don't think that there is a medical reason that would prohibit the majority of divers from being able to equalize in this position. I know that I have flipped onto my back to watch divers above me during a descent and I have never had a problem equalizing in a similar position in the water column.