It broke because it's a tired piece of shit thats been outside for years..plain and simple
nose clip a bolt, and leg stomp the 'biner with a long runner.
It's scary how easy a 'biner can break, if the gate doesn't close. Or has a 1/2 inch groove.
fixed draws are gay bad anyway...but this one probably sufferd a break during gate slap.
The QA lab at BD would look at that thing and laugh...and tell you the same thing.
I could agree with you, but then we'd both be wrong.