I'm late on this thread too, and what I'm about to say might be of absolutely no use, but...

You said that this is a recent conversion; are you new to fixed gear altogether? Was this your first time riding in the rain? Are your tires thin and/or slicks? If so, you might have just been skidding.

I've got two fixed gear bikes: an IRO Mark V and a converted Schwinn Sprint. For some reason (probably due to the differences in geometry (the Schwinn is really big on me (I found it in the trash))), my Schwinn will skid with very little backpressure on the pedals in the rain. I can even skid with ease while seated. It feels a lot like coasting, especially if one has become used to riding fixed and hasn't actually ridden a free-wheeled bike for a while. Longer cranks, which are probably what you have on your conversion, will also make skidding easier.

My suggestion: once you make sure everything is mechanically sound, dry reproducing the coasting when it's dry. Also, try locking the cranks the next time it rains and see if the resulting skid is the same coasting sensation that you're talking about.