Gate agents and F/As have been very militant lately when it comes to carry-on baggage. I've seen this on DL, UA, and US. It all begins at the gates with scolding, nagging-like announcements. Then the F/As never shut up about it during the boarding process. All this scolding needs to stop.
The airlines have done this to themselves by charging ridiculous bag fees, causing everyone to cram as much carry on luggage on board as possible. Every flight I've taken on WN, who doesn't charge such fees, has had plenty of overhead space when boarding is complete even on full flights.