Gopher population is highly spring weather dependent, in drought years gophers thrive but in wet springs they don't. This is also a localized phenomenon and just because one area has a large population doesn't mean it's a the same a few miles down the road.
I was told by a biologist once that during a wet spring when the gophers can't get dry for a few days they can develop a bacteria on their skin that eventually kills them. Drowning in their holes can happen and also, a spring thaw followed by snow and a freeze can seal off the holes and suffocate them in their burrows. I'm inclined to believe these theories since populations do so well in drought years.
Two years ago in one particular field that I shoot in, we had 8 inches of fresh snow on the ground and when the sun came out so did all of the gophers and I fired 600 rounds that afternoon (missed a few) and probably could have done the same the next day if was able to get there. That was a dry year and I've never seen that volume of gophers there before or since.