Sounds like bacterial conversion of CYA into ammonia as happened to me and that I describe in this thread (this post has a summary of chlorine usage before I got a reading and is described technically in this post). For every 10 ppm CYA degraded it would take at least 25 ppm and possibly 32 ppm chlorine to get rid of it. So 50 ppm CYA could take 125-160 ppm chlorine and that's not counting what is needed to get rid of any algae.

The best way to minimize the likelihood of this happening or of opening a pool to algae is to close the pool when the water is cold (below 60ºF and preferably below 50ºF) and to open the pool early before it warms up (so before it hits 50ºF or 60ºF). Of course, if one can maintain chlorine in the pool over the winter, that will work but isn't viable for pools that are closed due to freezing.