You can lose your spot in ultimate if somebody in Forest Arena thats is not in the top 10 ranks succesfully beats in in the Forest Arena. Then they automatically take your spot in the ultimate arena and you are back at their place in Forest Arena. To get back, you must defeat someone in the top 10 for Forest Arena again.

No matter how far up in Ultimate you go, people in Forest Arena NOT in the top 10 can challenge you as long as your Forest Arena rank 5-10. To avoid that, you need to defeat someone ranked 1-5 in Forest Arena while moving through ranks in Ultimate Arena.