good shots and good start.
1) good sharpness. Watch your framing.. clipped the top of the hat and the legs of the horse. Considering you shot at 210mm @ 1/160 with its pretty sharp.
2) Again.. framing. Exposure is good
3) Clipped off the rider. 1/250 seems to be too slow of shutter speed for a horse jumping at speed. Deciding how fast of a shutter is required to stop motion is definitely an art learned through experience.
4) A little blury.. from what I can tell.. a little of it was from handshake. I do like what you were trying to do with the composition. I like photos (portraits) of horses that show the ears forward... kinda hard to capture and a lot of it is lucky timing.
5) Now this shot is really good.. the best of the series. Shot at 1/320 f/4.5 at 28mm focal length. Has a certain journalistic flare to the photo especially with the other rider in the background. The horse is positioned nicely in mid jump with the front hooves already clearing the fence. There is a tiny bit of blur to suggest movement. You should try to remember this shot and connect in your mind the 1/320 shutter speed @ 28m focal length with the speed of the horse. The more you shoot the more you learn to guess the minimum shutter speed you'll need to either completely stop the movement or allow for a little motion blur. It takes A LOT of practice and experience... I'm no sports photographer so I am still trying to get a hang of it.
I also noticed all your shots were shot at 100ISO. Canon's are known to control grain pretty darn well... don't be afraid to bump it up to 400ISO or even more. This will allow you to shoot at an aperture smaller than most of your photos ( you shot mostly at f/4 to f/5.6) and/or shoot at a fast shutter setting. Faster shutter setting to stop the action... Smaller Aperture for more Depth of Field.