Braking bumps are caused by plenty of things, not just numpties. Natural trails are rarely natural - they are usually maintained by the sort of Trail Pixie activity we are talking about here.
Cutting corners is a sign of poor bike handling skills,Strava desperation or, very occasionally, a need to change the route a little.
A few things can help. Deberming is really helpful, and you can do it with the edge of your shoe if the stuff building up on the off slope is fresh enough. Clearing sight lines can help, too - just be stealthy. Cutting narrow drainage channels off a trail can do a lot of damage - wide shallow channels are good, so think about moving earth across, not cutting in or down.
Building berms, jumps and the like can be problematic for the landowner, and the easiest, safest thing for them to do from their perspective is to remove potential hazards.
If there is a volunteer group in your area, make contact and volunteer.
Hope all this helps. If you get the chance to talk to a professional trail builder, ask them how it is done. They may be involved in building trail centres, but they will be a mine of experience when it comes to maintaining 'natural' trails.