You've been riding for 51 years and never heard of this??? Have you never ridden in a group that understands safety and proper bike etiquette???
Proper safety protocol when riding in a group is to warn about hazards and also to warn fellow riders in the group to be aware of their surroundings. Yelling bike up, as some have already pointed out, is to let fellow riders to either not swerve into your path or to let other riders to behind the leader to single up should they be doubled or tripled. This is no different than yelling car up or car back when group riding. Yelling bike up wasn't directed at you just as yelling and pointing to warn about a branch or other debris isn't yelling at the hazard, it's a warning to fellow cyclists. As for the line in the middle, often on bike paths cyclists may ride near the line and even cross it if it's not too crowded.
Ultimately, whining about something you should have known after 51 years is ridiculous, (unfortunately there's no agreed upon name for a male-Karen), and "the head Karen", doing what was absolutely proper, might have saved YOU from an accident.
"Tree down up!" is perhaps my favorite road warning yell. Of course everyone behind knew exactly what I meant.
Perhaps the best way to understand why people say what they say is to pause after saying something and ask yourself, "why did i say that?" If the answer is that you are always logical and reasonable, have no cognitive biases, and empathize with everyone, then you are very unusual or more likely incorrect.