Ive seen this happen twice before. Once was on my buddies banshee, the throttle cable was wrapped around something and sticking, after we played with the throttle cable and gave it some slack it was okay. The other time was years ago when I first started getting into banshees. My dad and I had just rebuilt his banshee and were in the process of tuning it when it suddenly got pinned. I think that was due to the choke tube not being installed so carb tuning was way out of wack. It was pretty crazy tho, kill switch off, spark plug boots removed, it still kept going, and to make it worse this was the first time it was started after being rebuilt, talk about a rough break-in.