We can fix this problem by decluttering sooner
I agree that mass decluttering is an issue. It's an issue caused by the decluttering not having been done in the first place; if a challenge is decluttered as soon as it's posted, there's no need to declutter it later, and it isn't decluttered at the same time as other challenges (thus meaning that we don't get the "block of red" that makes the sandbox hard to use).
At present, I think there's some reluctance to decluttering other people's posts. If we have an explicit rule that "as soon as you see a deleted Sandbox post for a recently posted challenge, you should declutter it", that would help the declutterings be spread out in time; the more people decluttering, the less "clumpy" the declutterings get. (Or to put it the other way round: the reason we get declutter clumps is that few people are decluttering and they don't do it very often, meaning that a large amount of clutter builds up in between each decluttering event.)
(Note "recently posted" here; if a challenge dodges a declutter for a long time, you should probably delay decluttering it until there haven't been any declutterings recently, as the OP suggests. Additionally, if a challenge dodges a declutter for a long time, it'll probably be several pages back, and thus decluttering it will be less important.)