I would have first assumed that there wasn't enough gap at all walls. I would always keep 1/2" and no less for gap. Did you go back and look to see if there was any gaps left on all edges? What is the humidity situation? Was there a leak? If there is uncontrolled moisture or humidity any wood floor, nailed or not, will fail in this manner if it keeps getting moisture and doesn't have enough room to expand. Natural wood floors, nailed or not, probably even have a greater potential for expanding than manufactured floors.
What is the moisture/humidity situation in the floor tht it is installed on? Do you have a vapor barrier and insulation? The worst case scenario is that you have a condensation or moisture migration problem contributing excess humidity and/or you installed the floor in the middle of winter and it swelled in the summer. Have you tried removing all the furniture, checking the side gaps, and cutting a new gap to relieve the strain?