This has very occasionally happened to me before with Cyanotypes and Arches HP 140 paper, usually when the print was left face up, very damp on a surface or tray. Although I don't know the exact reason, I concluded it was due to washing as I often make test prints where I cut a large sheet into smaller pieces and the occasions it happened, the other pieces of the sheet were fine, and it affected the part of the paper where there was no sensitiser too.
Originally Posted by Tom Taylor
I always just use tap water, and suspect it is a result of the chlorine (which can vary in level slightly) reacting with the buffer/chemicals in the paper. Although manufactures say 100% acid free, unbuffered, free from chemicals etc etc etc almost all modern papers in reality have something in there. You can usually see this by wetting watercolour paper for a long time then shining a bright light through it - you will the stuff they have added show up as white splodges in the paper which won't be visible if you again shine a bright light thought it when completely dry.
Too much tween-20 (if you use it) in the sensitiser solution can create a similar effect though it looks much more pronounced than your example.