And that's a perfectly legitimate reason to do it. The problem being that they are taking away "All' guaranteed money and nothing is specified on what it means to "default" is the issue.
If they want to say "can't ride an ATV or ski-doo", fine. If they say, "If you default on your contract, you will lose all your guarantee" without laying out what it means in that same clause, they are basically trying to make it so no matter what, if something happens to him, they can cancel all of his guaranteed money.
It's the same type of "catch-all" that you see in job descriptions where it says "and all other responsibilities". You might not know you are responsible for something until 4 years later and your boss finally says, "Oh btw, you have to work this weekend because that falls under your job description". It's a shady way of doing business.