I'm somewhere in the middle. On the one hand, he forgot something and was technically no longer "in line." On the other hand, I think the aisle is for walking and people who want to stay still (because they have to put on their coat, gather their things, whatever) should stay out of it until they're ready to move. It's kind of like camping out near the doors and not standing aside to let people enter or exit the train.
I think there's entitlement to the attitude that everyone else needs to go at your pace, whether you want to go faster or slower, which is why I'm a fan of folks who want to go slower moving aside so people can pass. We've all been too tired to hustle after a long day. We've all had our planes arrive late and needed to haul butt to make a connection. We all know someone who has had their headlights suddenly go dim or lose tire pressure and not been comfortable going the posted speed limit. We all know people who have gotten a call that a loved one had an accident and is in the hospital and wanted to get there as soon as possible. Being tailgated is unpleasant, but so is being stuck behind someone who is going below the speed limit.
My personal rule of thumb is that people who want to use a space for its primary intended purpose have priority over those who want to use it for something else. For instance, let's say that there is only one sink in the bathroom. Its primary purpose is hand-washing, so people who need to wash their hands should do so. Even if I was there first, I'm going to yield to the lady who wants to clean up if I'm using the space to touch up my makeup. Doors are for entering and exiting a room/building/train, so I should step aside for the person who is getting in or out if I'm waiting for my cab or having a chat away from the crowd.
I agree with the PPs that you shouldn't risk toppling over to let someone pass. (Though if you are really very unsteady, I recommend staying seated until the train is completely stopped, not just from an etiquette standpoint, but from a safety standpoint as well. If the train braked harder than usual and you lost your grip, you could fall and hurt yourself or others.) In this case, I think an apologetic smile and a quick explanation that you aren't very steady probably would have