Bee Flies hop and hover while laying eggs

Bee Flies (Hemipenthes celeris) lay eggs in the sand and occasionally flutter around at Brazos River Park, in Sugar Land, TX, USA. These Flies have been quite a common sight lately in the area, but I've never seen so many together as I saw along this small trail near the highway. There must have been at least a hundred or so. I wonder why this spot in particular was so attractive to them?

I thought it was interesting that some of these Flies are so much bigger than the others.