What causes a yard to become lumpy and uneven. has just happened over the past few years. how do you correct it? thx
There could be various reasons. A photo could eventually help.
The most common problem (in my opinion): bad preparation of the lawn. Like road, the soil need to be compressed and well prepared, so that it should remain mostly even. Possibly with more compression on lower part, before to put the good soil.
Usage is also a problem. If you walk always in the same place, or you put the car in the same place, the pressure will more soil on the side. There is not much to do (but if you want to have the parks with "do not walk in". [because it is not a part, there is no tragedy of the commons, so ... not a good solution];
Water is also a problem. Soil is in any case never static, it change, geology (and hydrology) makes movements of soil (and they more away stuffs). If you think there could be such problem, first thing to do: check your home and fundaments. Speak with a construction worker about solution [remember that you have an house, so any solution should not move the problem from lawn to the house].
And possibly many other problems. A photo or more description could help.
What do? You can put some soil, sand on the "holes", usually in winter or when you use less your lawn. Not too much, so that existing grass will survive, but so that the soil will be more even. Rain and snow will help to make lawn even, by moving some of your added soil. You may do this twice every year. On next years usually you need much less soil.
If it is very bad, redoing the lawn is the simpler and quicker way (but only if you know the original cause and you solve it, e.g. bad preparation)
Freeze/thaw will make a lawn uneven . In locations like garden zone 5 ,it is common to "roll" lawn in the spring to flatten it. Typically an 18 " diameter steel roller is partly filled with water ( full is too heavy) and pulled over the lawn with a riding mower. I did it a couple times when a neighbor lent me his rig , but generally I was not that interested in lawns. It does make a smoother ride on a riding mower for the rest of the year.