It may be you caught the cable on a branch and it has been pulled and is holding the throttle open. Trace the cable down from the throttle housing and make sure it has not been pulled tight.
A sticky throttle can get you in trouble fast. It is best to make absolutely sure there are no problems.
I think you should be safe and make sure the cable and the throttle itself are free.
Go ahead and take the cover off the throttle, loosen the cable and remove it from the throttle cam.
Make sure the throttle is smooth and effortless when pressing the thumb lever. Now would be a good time to clean and lube with light oil.
Make sure the throttle cable is loose in the housing. When they get worn or dirty the cable will stick in the housing and be hard to move. You may have to take it loose on both ends to make sure, a pain as it is at the throttle bocy. If it is hard to move you may consider getting a new one.
From your explanation regarding letting the thumb lever snap back, it sounds like something may be dirty and tight in the throttle.
It may just be out of adjustment, but usually the cable pulls a little when you rotate the bars from left to right. If it does not change rpm when rotating lock to lock, it probably is the cable or throttle dirty.