War of the nest boxes

For many years I've enjoyed watching the Blue Tits as they nest in my boxes and there is one box that seems to be a particular favourite for them though I have no idea what makes this particular condo more desirable than any of the others.
Anyway I was outside preparing the new flower beds today when I heard birds squabbling at the 'particular' nest box. It seems that a House Sparrow had decided he liked the look of the box and had dared to land on the entry hook - what a furore that started! The female Blue Tit literally attacked the Sparrow (who is 3 times her size) while the male Blue Tit appeared to come from nowhere and dive bombed it. For a few minutes the Blue Tits and the Sparrow were fighting intensely because the Sparrow kept flying back to the box and so many times I saw the 3 of them hit the floor.
Eventually I intervened ( I know, I know I should have just let nature take its course) by walking over to the birds, the Blue Tits flew to a tree next to me while the Sparrow hightailed it out.
I was concerned for the female Blue Tit because she didn't look right so I sat and watched her for a while until eventually she just fell to the floor. I love those birds and I couldn't just leave her on the floor because the dogs and cats were out there too so I gently scooped her up and encouraged her to perch on the nest box.
The poor bird was knackered I think and it took ages for her to actually step onto the nest box hook but then the most amazing thing happened. Thinking I had done all I could I turned to walk away and the tiny Blue Tit flew over to me and landed on my shoulder, not only that but she started chirping and singing away.
I stood there for a while thinking "kkkkkkkay, what do I do now" and I didn't think she'd stay there while I fished my camera back out my pocket (I'd took pictures as I'd encouraged her into the box)  but she did - bless her!

I sat with her until she was ready to fly off to the tree again but no sooner had she got to the tree and that Sparrow came back. Both the Blue Tits flew off but the Sparrow stayed in the tree.
I don't know what it is about that ruddy nest box but that Sparrow guarded that box all afternoon just watching it from the tree never attempting to go in it and I must admit I got really worried about the poor female Blue Tit, I feared she may have had some injury I couldn't see, but thankfully both Blue Tits came back a little while ago and again as soon as she tried to get in the box the Sparrow attacked so another fight ensued, this time the Sparrow actually went in the box.
I don't know how this will end now but I know next year I'll make sure there's plenty of boxes for all and I'll be making sure the holes for Blue Tits are too tiny for the Sparrows - the Sparrows will have a terraced block.

I've never seen that kind of bird behaviour before and have never seen the Sparrows even attempt to go for this particular nest box but I think the opening hole has gotten larger over time and it's only now suitable for them to get in.
Even the Great Tits who already have a nest box have been hovering the Blue Tits box all afternoon and looking inside and I haven't a clue what's actually going on.
Is this normal behaviour for birds? Have the Sparrows maybe lost their own nest so are after pinching off the smaller birds?