Paint Stripping

Ahhh! What the hell?! I’m working on my grey scale today, which is a grisaille made up of my 40% linseed oil/60% mineral spirits mix and black and white oil paint, and as I painted the chest it started stripping off yesterday’s underpainting in the lighter areas. I really cannot understand why this is happening, and only in this one place? It’s a complete mystery as my technique has not changed. Good thing this is an experiment!