Some carriers are getting strict on the weight of all bags
It may be true that heavy carry-on bags can be a danger to passengers if they fall out of the bins, and the bins themselves are designed to hold certain proportions, but more and more passengers are getting angry if they are told their carry-ons are too big. A number of international carriers have no weight restrictions, but most do.
Confusingly all airlines seem to have different limits. Hawaiian Airlines is the only US carrier to enforce a 25-pound (11 kg) weight limit for carry-on bags, and gate personnel now weigh them at check in or at the gate, punishing passengers with fees if bags are too heavy. US Airways has a 40 pound limit. ANA limits carry-ons to just 5 kg, Virgin Atlantic to 6 kg, Emirates to 7 kg, Lufthansa to 8 kg. But not all airlines enforce the rules as strictly as Hawaiian.