Why does peanut butter become very hard when adding egg yolk?
Have you ever added liquid straight to tahini when making hummus?
See this quote from Cook's Illustrated:

'You’d think that adding a liquid would thin tahini rather than thicken it. Why the opposite? Tahini is simply sesame-seed butter, made by grinding hulled sesame seeds into a paste. Much of its makeup is carbohydrates, and when a small amount of juice (or any water-containing liquid) is added to tahini, a portion of each carbohydrate molecule is drawn to the water. As a result, clumps of carbohydrates appear. As the amount of water is increased, more clumps develop, causing the tahini to thicken overall. If you keep adding water, eventually you’ll cross over the threshold of thickening it; enough water in the system will cause the tahini to loosen and thin out.'

I believe the same is happening to you with peanut butter, and you would probably find it also happened if you added water rather than egg yolk.
I could be wrong...