Lacey does that too! She also does it around feeding time, usually after she's been worked but sometimes she does it when I'm feeding her and haven't made her do anything first. I just think she's yawning.
I read somewhere once that one reason horses yawn is to release stress (not sure if I agree since I've seen plenty of stressed, non-yawning horses, but maybe they weren't releasing it enough? I don't know) and that theory kinda fits with when I've seen Lacey and other horses yawn. Usually it's after a time of low-key stress.
For Lacey, I figure that she's just kinda shrugging off the stress from being ridden and relaxing because she's home and she's about to be fed.
Legacy could be releasing the stress of "When am I going to be fed? Now? Now? Now! She's here! I'M SO EXCITED!!!!" with his yawning... Also, Legacy is a gorgeous boy! Lacey wants him to be her boyfriend. Haha!
For Mac too, the need to be patient when he doesn't want to be patient is most likely a source of stress. I know it is for me in my life! Haha
I don't really know for sure, obviously, but that's just how I interpret it.
Alternatively, we can just interpret it as them being ridiculously adorable!!