The freezing and thawing of the ground is pushing the rock up. Here in northern MN it happens all the time. Especially after a really cold winter when the frost goes deeper than normal. The most noticeable rock issue I dealt with was in my back yard. In 3 years time it went from a half dollar size bare spot in the yard to sticking up high enough that I had to mow around it. The next spring as the frost was coming out and the ground was really soft me and my oldest daughter were able to pry it out with a couple of crow bars. It was about the size of prize winning watermelon.