I only recall seeing this happen after combat is concluded. I think they are trying to point out that combat is stressful and tiring, which makes sense to me.
Are there other scenarios that you just see it happen randomly? Are you carrying a lot of barrels and jogging around Faerun? I know that tires me out when I do that around here - the neighbors have questions.

It encourages the player to rest so certain Cutscene and plotpoints can be activated.
I’m not sure there’s a pattern. I think it’s been theorised that it appears when someone has something to say back at camp, but I’m pretty sure that I’ve seen it when that’s not the case.
Thank you all for your response regarding the tiredness mechanic in the game. It's interesting to see differing opinions on when and why the characters mention their need for rest. While it's possible that the developers intended for the mention of tiredness to be related to combat stress, it seems that this is not entirely clear or consistent. It's also possible that the mention of tiredness is simply a way to encourage the player to rest and trigger certain plot points. Regardless of the reason, it's important to note that this mechanic doesn't seem to be entirely understood by the player base, and it would be helpful for the developers to provide more clarity on its purpose and implementation in the game. Alternatively, it seems we`ll just have to wait until the release. I'm quite certain that what we are able to play right now pales in comparison to how the official version will play out.