Anyone know the reason?
I tried to search for some clues on Decred's official forum, here:
https://forum.decred.org/But there is not much activity going on there. Barely any recent discussions.
So without any serious news, it is safe to assume that it's standard pump&dump action.
No need to assume. It is surely a pump. We have seen this occur many times in a cryptocoin's trading cycle. It will be pumped and then dumped then pumped again to the traders delight. When there is no more bitcoins to be made because the newbies ran out of money they leave. Then the coin fades in trading and sometimes delisted by the exchange because the volume is very low.