While carving a relief, my machine suddenly cut deep

I'm new to Carveco and I just tried to run a relief. I used on of the reliefs that comes with the program, bird in a circle of leaves.  I set it up on a 5 x 5 x .75 piece of plywood. First I did a roughing cut with a 1 inch bit and it ran fine. I did a second roughing cut with a 1/8 end mill and that ran good. I started the finish cut with a 1/8 the ball nose mill. It ran great for the first half of the relief, than suddenly it started cutting about 1/8th inch deeper. It kept cutting the relief ok so i let it run. Thane about another 1/3 of the way up it dug in again and was taking about 1/4 cuts with each pass, so i stopped it. Why would this happen?