It rather seems that the beginning of the end was the desperate so-called "deal" done with GM. It's nicely, or brutally, summed up in a Seeking Alpha article [1]:
The deal shows Nikola doesn’t have any technology of its own, be it battery technology or fuel cell technology.
The deal also shows that Nikola is willing to forego any possibility of an economic return, just to further the story and pump the stock.
Arstechnica also has a piece [2] today about the deal.