Look very carefully at the pattern of where exactly the grassy weed appears. If it is in patches or slightly different where different truck loads of topsoil were brought in, there is your answer. HOWEVER...If it occurs uniformly across every area that was seeded. It probably came from the seed itself. Or from the straw or mulch. Wheat is possible if wheat straw. Can you identify annual ryegrass? That is my guess.
I had a customer redo his yard due to crappy seed. Roundup --rototill--reseed--and he planted crappy seed. Can you believe it! Lots of annual ryegrass. Tall, wide blade, yellow color, thick base of plant with distinct red color at the soil level. Also long clasping auricles.