I actually think it's Larian's implementation on the combat facing rules (from chapter 8 of the Dungeon Master's Guide). Basically, shield bonus only applies to attacks from the front or the same side of the shield, and attacks from behind have advantage.
It's an obscure variant rule that few tables actually use, but it's technically a legit D&D 5e mechanic.
A key part of the optional facing rules is that creatures, as a reaction, can rotate themselves in response to enemy movement and thus prevent backstabs. Additionally, at least how I'd read it, when you move from the front of an enemy to the back, you'd provoke an AoO because you are moving out of the enemy's reach/threatened zone.
