How Is This Happening?

Poor Rogo. Doug says he is accident prone. I have to admit, he was very clumsy as a 3, 4 and even 5 year old, but he's much better now. Anyway, Fri. evening when I went to groom and tack him up, after a week of so much rain I'd only gotten one ride in, I found his right front leg bleeding from a small cut. The weird thing is, and I'd sure appreciate thoughts / ideas on this, is that it's the THIRD time he's cut this area on a front leg and all have been a mystery as to cause. The first time was the left front, then a few months later the mirror image of this cut appeared on his right front, and now, two years later, the same cut is back on his right front.
The vet said he'd never seen this before (after the second one appeared) - two identical cuts on opposite legs, occurring when Rogo was in  different locations (so it didn't appear to be his environment), and no visible cause. We even wondered if Dan, the Appaloosa, was biting him in play, although it didn't appear to be a bite mark. Now Dan isn't here (he's at Joan's), so he didn't do it. Savanah is the low horse on the totem pole and wouldn't bite (and it isn't a bite). It isn't a kick. It presents as sort of a puncture / cut. Here is a picture of it:

Could he be hitting it and slicing with a hoof when he's getting up? I've watched him rise, with this question in my mind, and he didn't come anywhere near his front legs with another leg. The first two times he wasn't wearing shoes, but now he is. There is a small nick in his left hind ankle that happened at the same time.
Although it is fairly innocuous this time when I found it right away, the first time he was at a boarding barn and I didn't get there from a Thur. to a Mon. and when I came on Mon. his leg was like a huge stovepipe, with rivers of puss coming out of it. He was off work for 6 weeks and went through two courses of antibiotics. So if I can find a cause for this and manage it in some way it would be a great help. It doesn't cause any permanent harm, but it sure would be nice to put a stop to it, or at least understand it.
Here's the care routine, so I'll remember dates and tasks. Every twelve hours:

1. clean the wound with a toothbrush dipped in warm water, to keep the would from sealing over with dried fluid - we want it to drain
2. insert a blunt syringe filled with saline into the wound, flush it, and squeeze out any / all fluids. Repeat this three times each treatment. Do this for three days (until wound is almost healed closed)
3. Give him 25 cc's of penicillin - a needle twice a day alternating neck and rump for five days

On the bright side, the vet said Rogo is well muscled and very well behaved, which makes treating him easy. He has to stay in his stall to keep flies out of his wound, but starting today I can let him out at night in their grassy area (not the sand riding ring).
We had a great time on our one ride - I did three loop canter serpentines twice. He did them easily - Yeah Rogo! Then I put him on a 20 m canter circle which I picked up on the long side (not a corner). As we cantered I pushed him into the outside rein and bent him slightly in - thinking I was doing the exercise from dressage camp. Guess what? He'd been counter cantering and did a perfect and instant flying change when I applied the aids! Wow - my first flying change since I was a teenager. In reading Kyra Kurkland's book I see that this is how she introduces flying changes - she counter canters a 20 meter circle and then asks for the change, so I think this won't be too hard for Rogo. I won't do it again though until I have some instruction as to when to teach it. I want to make sure I don't screw up his counter canter, although he seems to understand the difference in being asked to counter canter and being asked to change, at least in this one instance :)
Remember - please send me your thoughts, ideas, experiences if you can help me solve the mystery of Rogo's inside front leg cuts. Thank you.