Top up skills? Are you kidding?
Recently I completed both the French & the Swedish course. I use Duolingo every day (currently on a 107 day streak) to top them up to 100% and am also chipping away at a few other languages. Most days I have to do 5 or 6 lessons on the French & Swedish to get them to full strength. Today I've gone in to find that I need to do 34 for French and 61 for Swedish! Some lessons have dropped by three points since yesterday. I haven't plucked up courage yet to look at the other languages. Why has this happened?
I think that your poor trees decayed so quickly because of a change in Duolingo's system. In this post from yesterday they explain that they improved the skill strength algorithm:
Apparently implementing the new system caused large numbers of skills to decay, but it is a one-time thing. So once you get them back to full strength, you shouldn't have to worry about the same thing happening again. Unfortunately the announcement became hidden rather quickly in the discussion forum. Hope this is useful!
The only thing I can think of is that when you first completed your tree(s), you did a lot of lessons on the same day (so they decayed at the same rate). But that doesn't really explain a total of 95 lessons to regild!! I'd be crying "foul!", also! It doesn't seem fair; also, a great de-motivator, for sure!