I could write a book on the things that eat at a lawn so I'll mention just a few and you can do your own research.

Moles and voles...eat at grass roots. Slugs and snails do the same.
Skunks will dig up a lawn to get at the slugs.
Mice move across a lawn in winter eating what they find.

If you have a slug problem try this: dig up a small section...about two feet square, look under the sod; if you have just a couple, don't worry about it...just pick them off and deposit into a bucket of soapy water. If you see more than 6...then action is called for. There are a number of methods of attack....do some reading on how to remove them.

Moles and voles are the big troublesome pests that can destroy large areas of a lawn. Do read about them too.

Dogs....urine. the female is the worst offender. If you have dogs of your own or neighbors' animals that visit, there is absolutely nothing that will cure the problem other than watering more often the place where they go...and replacing the sod with plugs, overlaying some topsoil and overseeding.
There is no need to go to the extreme such as buying an electric fence, or laying down hot pepper that, if the animal gets it on his paws, then wipes his face, gets it in his eyes...and blinds him. Water from a hose is the usual shoo that works. And reminding his owner that any damage done by his dog will be followed by a bill in the mail for damage and repair.

It sounds like it could be moles. They are mostly carnivores looking for earthworms. I have not seen any literature stating that they eat grass or its roots. They tunnel under the turf and this action pushes up the turf causing turf grass's roots to lose contact with the soil, the turf suffers and can die. There are people who try to flood them out. There are all styles of traps available from humanitarian to deadly. there are also a number of poisons, some resemble earthworms made out of jell. Although blind, moles are not stupid, none of the traps or poisons or attempts at flooding are very effective if not completely worthless. I can't say it's related (cause and effect), and I'm not recomending it, but I haven't had moles since I started aerating with a 1700# tractor. On the otherhand, it could be because I just don't have many worms.