The reason he is getting more aggressive is because you did nothing to punish him for being aggressive the first times he threatened you. Horses never do things like this to begin with. They do little things that show disrespect. And if a handler does not put them in their place (below the handler on the pecking order) they get a little more disrespectful and more and more and more. This can go on for quite a while with some horses. More dominant horses may step up aggression pretty quickly.
Horses give off small signals to start out with. An experienced horseperson will see a little toss of the head or an ear flicked back or just a step toward a person and will immediately make the horse back up or get over or do something to 'yield ground' to the handler. Now, if the handler, instead, backs up a step, the horse will consider it a complete surrender.
A horse that is dominant by nature will very quickly 'up the ante' and try to 'drive' you out of their herd of two. This is what this horse did when he ran up behind you and bit you and knocked you down. He just reaffirmed that he was waaay above you on the pecking order.
Just watch horses when someone puts out feed. The 'lead' horse will walk up to the first tub of feed and all other horses will walk away. If they do not yield ground quickly enough, that boss horse will dive at them and take hunk out of their butt. That is what he was doing to you.
You need to work this horse -- hard -- and make him move his feet when and where you want him to move them. Then, more important than anything else, you need to make this horse back up while you very aggressively make him back faster. Backing up is a complete expression of submission. [This is how he takes you backing away from him, too.] You need to do whatever it takes to get this horse's respect. You MUST be at the top of his pecking order or he will get more and more dangerous.
If a horse surprises you with an act of aggression, run at him, scream at him, throw whatever is handy at him, chase him all over the pasture. I've thrown bucket of feed, snow-balls and chunks of ice, clods and sticks at a horse that dared lay its ears back at me. You can bet that he knew it was a mistake even if i did not have anything at hand to really punish him with.