Why is this a blunder?

The computer states that a4 is a blunder, what did I miss?

Engine is pointing out h3 as the move. Looks like the black queen is close to getting trapped, maybe something like h3 Qh5 g4 Qxh3 Ng5 Qh6 Nxe6 and after the queen moves again there is Nxc7+. 

Interesting, I haven't considered that line of play. I guess I was too focused on how black can punish me for the move to account for a missed better tactic.

Thanks a lot!

👍 engine assigns these terms based on the jumps in numerical evaluation. a4 is not a blunder in that it loses material or is even a bad move in itself, still showing a big advantage to white. Its just that there is some line there starting with h3 which gives a winning advantage so any move you would have played apart from h3 would likely be classed as a blunder. 

yes, when a side plays a move that decreases the evaluation for them by more than 2.00 I think it counts as blunder to the computer

This is a good insight that I'm sure many players were unaware of in that an opportunity missed is deemed a blunder. Well explained Strangemover. How many times do we oversee a wonderful missed tactic that, in hindsight, wasn't overly complicated?!

I also won't think it is a blunder if I played it, but in terms of material maybe it is blunder. By the way as I see engine play h3, it began more clear to notice the black position is fragile😂 h3 is a very excellent move!