Crab grass is an opportunistic weed
Crabgrass is an annual grass whose seeds remain in the soil and can stay viable for some years, waiting for the right conditions. It appears late in the spring as the soil warms up, late May and June. Proper maintenance is the best defence against it. Cutting the grass too short, heat and drought as well as thin grass all favour the germination and growth of crabgrass.
Other than hand pulling, crabgrass can be treated with a corn gluten based weed preventer that blocks annual seeds from germinating. Results have been variable so far. Once germinated fiesta weed killer has shown some effectiveness on the young plants. We use both treatments in susceptible areas.