If I had to guess it's because the system is temporarily pausing tdd wifi traffic while it scans the 2.4 and 5.x GHz bands to see what SSIDs are broadcasting.

If I had to guess it's because the system is temporarily pausing tdd wifi traffic while it scans the 2.4 and 5.x GHz bands to see what SSIDs are broadcasting. It is a bit of a trade off since if you want to see every possible available AP, even the shitty ones with signal levels at like -80, you can't be noisy on your own radio at the same time as you scan the band. Remember it's a half duplex medium. It does it even more if you hold down option and click the wifi menu bar, to get detailed signal strength/info on the AP you're presently connected to.