Because it was cold, so every animal stayed in bed and cuddled...

That is essentially the hypothesis they want to test:

The increase in viscosity—which increases as water temperature falls—could yield an evolutionary advantage to those single-celled organisms that clumped together, using their combined propulsion efforts to their mutual advantage. In his laboratory, Simpson and colleagues have found that a type of green algae responds as he hypothesized it would.

When everything is easy, nothing happens.