It is difficult to determine the cause without knowing your location or other possible factors. You can help prevent this in the future by taking a few steps. First you should aerate your lawn to help the natural grass grow better. This should help prevent the clover growth. Fertelize in both the fall and spring seasons to keep your natural grass healthy. Make sure not to cut your grass to short. Leaving the natural grass a bit longer can help prevent unwanted weeds/clover from sprouting. You can also try to apply a comercial treatment product for clover. This should be done in the early spring before the clover have a chance to sprout. Waiting to long to do this will allow the clover to sprout and will make the product less effective.