a first and a first and what did I do wrong?

Today actually took Thump off property, in trailer, with others, trail riding to a local state park about 30 miles away. Was VERY hesitant about doing so but decided if I kept putting it off, I'd never do it so off we went. Buddy and D went also. Ted, former owner, rode D and a friend of theirs rode Buddy. When we got there and I looked around, I realized that all 5 of us riding were there that fateful day back in Oct 2005 when Thump took off on me the first time I rode him. I started to have a bad feeling but decided that I needed to put it away or Thump would feel it too. Get Thump unloaded and he's sweaty and has diarrhea from the ride over. Great. By the time I have him saddled and ready to go, he seemed calmer and I felt a little more confident. Off we go. The trail was beautiful and we had a great time. Right up until about half way thru when we were going up a steep hill and I was leaning forward like taught when going up hill and suddenly I heard a loud noise and everything went black. Thump, even in his tie-down, flung his head back far enough that he popped me right in the face and altho I didn't pass out, I couldn't see anything for several seconds then saw just stars for about a min then just saw blood everywhere. By the time I could see again, everyone was stopped (Thump either stopped on his own or I stopped him, no clue) and someone was standing next to me handing me tissue for my nose. I have a deep cut across the bridge of my nose that bled for about 5 or 6 hours that must be from his bridle and a bloody nose. Did not break my nose, nor did it even hurt (till about an hour ago), only my head hurt...massive headache, which I still have. So we finished the ride with me stopping every few mins to wipe blood off my face. Got back to the car and someone had a first aid kit and cleaned up and put on a band aid then continued on another trail. However by this point, I think I was about done and stressed out and Thump sensed it because we started having trouble that started with him refusing to walk thru a gate and ended with him almost flinging me into said gate. I accept full responsibility for the problems on the second part of the ride and knew it wasn't going to get any better because I was just mentally and physically done for the day so I dismounted and walked him back to the trailer.
What I don't understand is how I got hit in the face. I don't feel like I was leaning too far forward and the hill didn't seem very steep compared to some we went up and he had his tie-down on, which I don't ride without. I know his tie-down isn't tight enough but he's never thrown his head around until today and after he hit me, he really started throwing his head around a lot. I always keep him at a walk and he was doing his fast walk/half trot today and I know he did some type of lope up that hill that wasn't a gallop but almost rocking like one and since he was under control at the time, I let him. So, what did I do wrong? This is something I really don't ever want to repeat.
All in all, I think the day was really good for us. It got us off the property into someplace new and with other horses. I rode the brake the entire time, but everyone else was too. There were 5 of us so maybe next time just a few less riders. I also got to see my first paso fino (pasa fino?) in person and he/she (didn't look) was beautiful.