Sparrows attacking another is this normal?

I have quite a large population of sparrows that visit my garden, even more now their young are also visiting, however the other day I noticed on my patio what looked like a dead sparrow on its back and another one pecking at it viciously.  I thought it had killed it and knocked the window to get it away.  To my surprise the other one wasnt dead, it was just completely on its back and not fighting back.  It flew off witht he other one close behind it.  It was quite a small sparrow, so not sure if its young or just a bit of a runt.

 A few days ago I was looking through my security camera footage which happens to look over one of my feeding stations, I was looking for some goldfinches that have recently started visiting and instead at 5am I spotted what looked like the same small little sparrow on its back again! this time it was on the lawn and had two other sparrows attacking it, each time it tried to fly off it was put back down again. They were attacking it for ages:( I thought it would die but it managed to get up and fly off.

Today the same thing happened, this small little sparrow flew into the garden and within minutes it was set upon by a number of others:( They were really going for it.  I opened my window and most of them flew off with the little one managing to then get to some food.

Is this normal? :(  Is it because they can sense something might be wrong with it? I feel like I just want to scoop it up and protect it but obviously I cant.

Ive never seen this behavior before.