Little ridges on butcher block style counter top

I made a counter top out of 8/4 hard maple stood on edge that was ~ 2.5 inches thick. I sanded it smoth and put on 6 coats of tung oil. A few weeks latter my smooth counter top developed small ridges at each of the glue joints. You can’t see the ridges but they grab your fingers as you wipe your hand over the top.

My question is what caused these small ridges to develop and how do I prevent them in the future?

Thanks for your help

-- Glenn, New Mexico