Dog Triggered By Random Phrase

I have two dogs; one a mostly lab 5 year old I’ve had since he was 8 weeks old, the other a rescue Pyrenees who’s almost 2 I’ve had since about 1. My 5 year old is triggered by the phrase “Good Luck!” (Especially when done with a thumbs up) to bark. I have no idea why, my wife and I found this out randomly one night but I was wondering if anyone could help me figure out why.

Some other background – 2 year old is not triggered by this but hardly speaks in general (only if startled). He barks off command sometimes, but if asked he responds to only 4 phrases (speak, boof, woof, and bark). He responds to good luck even if it’s not directed at him. Off command it’s typically reacting to something he’s overly excited about. Hes very smart and initiative and is well behaved for the most part as well.

Any insight id love to try to crack the code to his brain lol