Because when an opinion on Twitter starts to “trend”, it can take on a momentum of its own, and people adopt it simply because it's popular, not because they really believe it.
And momentum means a force that keeps something going once it has started.
An example of momentum is how quickly a car is moving down a hill.
An example of momentum is the encouragement of your parents, which keeps driving you forward and encouraging you to continue to succeed.
The players seemed to lose momentum halfway through the game.
gain momentum
As the days passed, our campaign grew in momentum.