This thread is peak RailUK.
Someone was wearing a rucksack - as most people do, because they have to carry things around - and was a little bit silly. They didn't try and cause a problem, or go around intending to **** people off, they just didn't consider that others may have been caught by it.
There are literally thousands of incidents like this caused by all sorts of things - not just backpacks - and it's an inevitable consequence of the fact that, if you want to use public transport, you have to share it with other people - no matter what some people on here may want.
As an aside, if I'm on a crowded train and I have my backpack on, I'll leave it on. Faffing about swinging it about to take it off and then back on again is probably going to cause more inconvenience than leaving it on anyway, and I'll usually (if possible) lean against a wall or post with the backpack against it.
+1. Incredible that people are asking if someone could be guilty of manslaughter by gross negligence. Peak indeed.