Once upon a time while living in Alaska, I was driving home one night on a "back road". There is a particular section on this back road where a Mama MOOSE and her twins would often cross. The speed limit was 25, but I slowed down to about 15 as I neared the "crossing". The car behind me started honking and was right on my bumper, I ignored him. Sure enough, Mama Moose ran out in front of me and I was able to stop in time because I had slowed down. She crossed, I waited.... Guy behind me starts honking again.... I held up my hand in that "Hold on a minute, please" gesture, and waited as Baby Moose #1 crossed, and then waited until Baby Moose #2 crossed, then I slowly pulled ahead and continued on my way, resuming normal speed.
When I got to the gas station and pulled in, the guy who was behind me pulled in, too and came over and asked why I had slowed down, and how did I KNOW those 3 Moose were going to cross right there. I told him I drove that way most nights and 9 times out of 10, the moose will cross. Plus, living just down the road, I KNEW that Mama had 2 babies.
I also told the guy that Honking when you see a Moose is very dangerous, as it may startle them, and a Moose CAN kick out your windshield.... Its been known to happen.
My point is, sometimes there is a reason for people to slow down, although you (general you) may not see it or know what the reason is. But in the case of the OP, if a driver is holding up more than 3 cars, its polite to let others pass if it is at all possible. Its even the law in some states, but I can't remember if it was 5 cars in Washington state?
I have been away from home for a while.