Your yard needs to have a certain grade, or slope, to ensure your home isn't besieged by water during heavy rain. The slope — and even supposedly flat yards have a tiny slope — allows the runoff to head to a storm drain, rather than your home's foundation. Because soil can move, however, you have to re-evaluate the grade occasionally and arrange for excavation and regrading if necessary.
After Prolonged Severe Weather
One bad thunderstorm shouldn't change the grading in your yard. But a prolonged drought that dried out your lawn, flooding that lasted several days, and even a series of severe storms that brought lots of runoff can damage part or all of the yard and lead to a ruined or changed grade. Parched soil that has cracked can become uneven, and even more so once the rains start up again and make the soil wet again after washing away debris. A flood can carry off topsoil, and heavy rains over several days can create little channels of eroded ground in your lawn. If you see any of these happening, you need to have someone look at your yard and arrange for excavation if needed.
After Seismic Events
If you live in a region prone to shaking, you know a small quake isn't going to do anything. But if you have a moderate or large shaker, take another look at your lawn even if your house is OK. Loose soil can "liquefy," changing the landscape in a few seconds. You'll need to regrade the entire lawn if you see signs of this, and in fact, you may need to have a company carry away small rocks that may have been pushed to the surface. Try to get this done quickly, or at least as quickly as possible given the effects of the quake on your city.
After Repeated DIY Re-landscaping
You've taken great pride in your garden, and if you're a DIY landscaper, you may have been working on your yard for years. Repeated digging for new beds, however, and new lawn seedings can also create subtle changes to the overall grade in your yard. Every couple of years, have someone evaluate the grade and then regrade around your house, if necessary.
Regrading your yard is not an easy process where you just take a shovel and move dirt yourself. You need a landscaper and possibly an excavation services to remove and replace the volume of soil necessary to create a stable, correct slope away from your house.