Crab grass is an annual and the seeds likely blew in this spring, taking advantage of the bare soil. It is easy to pull up if your plot is not too big, pull it before it sets seed, then mulch.

The grasses are slow to germinate and you should have either mulched heavily with straw or planted a nurse crop like rye along with the grass seed.

Prairie Moon Nursery has good instructions on starting a prairie from seed

Here is a link that might be useful: Prairie Moon Nursery

We had the same problem with our mix of buffalo grass, blue gramma, and side oats gramma being overrun with crab grass. I thought our meadow was a failure. Our solution was to keep the area mowed to a height of 6-8" and then do a burn the following spring. That was two years ago, we burned this year also and there is not a hint of crab grass and very few other weeds, which we pull as we see them. The burn seems to encourage the native stuff while destroying some of the non-native seed bank. If you're able to burn, it's the best way to go, in my opinion.

Thnx Jim-
I cannot pull the crab as it has very thick, fibrous root system [Judy B]. A friend has suggested, since this is an annual grass, also mowing off the seedheads and burning to kill remnant seeds. I presume this will not endanger the existing LBS, SOG and forbs as long as they are dormant??

More likely, the seedbank of crabgrass was already in the soil.
Most restorationists will tell you that any parcel of land should be prepared for at least one full year and preferably two, prior to seeding. Any area that has had crab grass or fescue or any other undesireable plant in it for a period of time will have a stockpile of undesireable seed on top of and under the surface that must be eliminated to ensure the natives have a chance to compete.
the key is to get as much of the unwanted seed bank in the soil to sprout and then kill it before it has a chance to go to seed itself.Keep in mind there is also a seedbank deeper in the soil so that disturbing the soil by raking or discing will bring more unwanted seed to the surface to sprout.
Poor site preparation is probably the leading cause of failure in a native plant restoration project.
As was mentioned here, mowing the first few years is also beneficial to your success.

Last summer, 2009, we planted short grass prairie seeds and prairie plant seedlings in a large area. Although the ground was compacted and there was no prairie grass from the seeds, the seedlings were thriving. Then crabgrass emerged and matted 1/3 of the space. I mowed it using the highest lawnmower setting to keep it controlled. This spring there was little to no prairie grass growth in the dead crabgrass area. In the rest of the area, little bluestem was emerging. In late June, this year's crop of crabgrass emerged in full force, matting a greater area of the new prairie. It was recommended to let the crabgrass alone this summer and next spring, we are to have professionals burn the prairie. Will a spring burn really prevent an annual, summer weed from emerging?