While this is unfortunate, and annoying for all concerned, it does happen.
Give a crew member in the vicinity of 12 minutes to board 190 passengers, whilst checking not only flight numbers but for hot beverages, mobile phones, electronics, cabin baggage, signs of intoxication, dangerous goods etc etc.... All you've got to do is add another flight with a similar flight number (such as the last number a 4 or a 9 for example) and a passenger who has simply followed the flock so to speak, as opposed to reading signs and listening to announcements, and wazzam!
A very unfortunate by-product of the whole "every extra minute spent on the ground equals so many thousands of dollars to the airline" mentality! Nothing to do with paying peanuts and getting monkeys, and everything to do with airlines adding enormous pressure on staff to work with stupidly short turn-arounds, scheduling down to minutes, and even 30-second blocks. OTP means everything.
Not safe, not perfect, not even desireable for our passengers, but try telling that to the bosses in the ivory towers....