Minor Question: Bees hanging from screen

One of the nice things about getting into beekeeping is learning to let the bees do their thing and chill. Very similar to my introduction to brewing 20 years back.. relax and have a brew.

I presume this is all in the same vein, but I am curious.

I have a TBH that got started a couple months ago and is going well. In the last 4 days or so, it has gone from a few bees wandering around the bottom screen to perhaps 800 or 1000 bees that are building up on the bottom screen.

It isn't overly warm (hitting mid 70s), it isn't particularly breezy. I have noticed the bees do have tended to stop spreading out even though the hive is doing well. A week or so ago I dropped 5 more bars into the brood area of the hive and immediately the bees started working them while continuing to cover what had already been built. I think they just don't like to spread horizontally as much as perhaps they do vertically. They seem to be sleeping on the bottom of the screen overnight as well, which reinforces my assumption it isn't about radiating heat.

Anyway, I'm wondering if a) they are maybe feeling crowded and rather than spread horizontally they just are spilling out of the hive or b) they are providing additional cover against ventilation by covering the bottom screen (as there is just screen the length of the hive.)

Currently they bees have filled about 20 bars, each of which has the equivalent area as a medium frame. That's roughly 2/3 of the TBH. I'm tempted to rout out grooves on a # of the remaining bars and toss a super on and see if that gets the bees a bit more interested.

Regardless, that is separate from understanding what the girls are doing and why they might be preferring to hang out under the mesh rather than in the hive.