For a few weeks now mine have been coming in loaded with pollen like that in the early morning, say 6:30am to 9:30am, then they seem to switch to nectar since I don't see any with pollen after around 9:30-10:00, though traffic is still heavy. And yeah, really heavy loads of pollen, big bulges. The hive is about 100 yds from my door so I usually walk out to see what they are doing from time to time. I guessed that the sun warms up flowing plants, nectar starts flowing, and the bees switch over. Dunno.
Are they carrying yellow pollen? Do you have Bahia grass growing their?
If so they are probably getting from the grass. My bees do that only in the morning.
What is supprising is that when you look at he seed stem of Bahia, it has green seeds with what looks like black pollen but they get yellow pollen from it.