From my last game; I'm trying to understand why the engine thinks this is such a bad blunder when it seems like all my pieces were defended, and I was adding pressure, a 3rd attacker, to a pinned piece? Appreciate any help understanding this.
As far i understand the position the move you needed here was a Knight move. By playing Bg4 you allow black to take them (with their queen) without any compensation after all captures.
So there's nothing like numerical superiority except the 2 vs 1 on your knight.

For the engine these are the kinds of situations where simplifying is more preferable than pressing.
From my last game; I'm trying to understand why the engine thinks this is such a bad blunder when it seems like all my pieces were defended, and I was adding pressure, a 3rd attacker, to a pinned piece? Appreciate any help understanding this.