Every ticket in every cabin includes a contracted checked-in and cabin baggage limit. Yes it may be unusual for cabin baggage to be weighed on full service airlines but none of us has a leg to stand on complaining if we are over the limits and those checks are made.

The weighing of cabin baggage on your flight may have been due to a number of factors affecting that particular journey on that particular day - stronger than normal headwinds, needing to take a longer than usual route to avoid weather or war, full passenger load, higher than expected cargo load, etc - any of which may push the aircraft closer to its performance limits on a very long journey such as LAX-DOH, making the weight and fuel calculations more critical than usual.