Why is Bg3 a blunder?

In this game the quick analysis says that white's move 11 Bg5 (sorry thread title mistakenly says Bg5) was a blunder -2.41.

I was playing black and I would like to know why it was a blunder since I obviously failed to take advantage of it.

My failure to respond with Bxd4 was a counter-blunder +0.23

But the queen would just have recaptured the bishop so why were either moves blunders?

Thanks JS.

All is now clear!

Note that the Bishop on g5 is undefended. Loose pieces drop off!

Note that 11...Bxd4 12. Bxe7 trying to save the White bishop from an otherwise possible 12...Qxg5 does not help things either, since 12...Bxc3 keeps Black an extra piece, and obviously 13. Qd8+?? is not checkmate due to the a5 Black queen protecting the d8 square.

I wonder what your opponent was thinking when he/she played 13. Qd6??.

Well, horizontal attacks like that are difficult to see, especially when the fourth and sixth ranks are occupied by pieces and pawns.

^Thank you for your kind words Eric!

I feel slightly better now.

>>I wonder what your opponent was thinking when he/she played 13. Qd6??.

I think they were tying to stop me castling my king to safety. I have blundered a queen before so I know how it feels!