"always"? no ... however, departing SEA in mid summer is often not a particularly fun travel experience, and the late morning time frame (for which people are checking bags mid morning) is exceptionally busy

not that this does OP any good after the fact, but curbside bag check is generally your friend (this is true pretty much everywhere, although we almost got bit at DCA last year ... at T-46 there was only one person in front of us, but that person took almost five min to finish)