Now...I'm new- but I know that two of the reasons that Finsky listed were not true.
The temps here when I put the 2nd deep on were an average of 90* day and 70* night.
As to nectar, I had 4 acres of clover planted all around the hives and all of that was in full bloom. And my neighbor's gardens were in full bloom. And they seemed to have no trouble finding nectar and pollen to fill any and all space in the frames they had already pulled out.
And as to them already wanting to swarm- the one that swarmed was a new hive of a Dadant package I hived on April 1st of this year.
I think that the reason mine were not filling out the new frames was either that I got some frames that they didn't like (I have replaced those with other frames and they are working them), or they didn't realize they were supposed to fill them as I put the new deep on the bottom and didn't checkerboard it.
I dunno. All I know is that it's been hot as hell all year and that those bees ain't seen a night below 65* since they arrived.