I think Twitter was stuck in a not-especially profitable niche. They shift into fast-mode to get out of it and find a better spot, then they can shift back into stable mode once they occupy a better equilibrium.
That said, there are lots of bugs in Twitter now, today, when they presumably had the benefit of being in stable mode for a long time. For example, Twitter regularly refreshes and loads new tweets while I'm reading them, pushing the tweet I was in the middle of reading out of view. That seems like a pretty silly bug to exist in a mature product. I regularly reach a state where I have to kill the app and relaunch it because all of the "back" commands just minimize the app instead of taking me back to the timeline. I could go on.