> Umm... why would you not choose a relational engine to start with?

There's typically two answers to this question:

When they first started things were changing so quickly they didn't want to commit to a schema and felt like doing real data modeling would slow their velocity down so they dumped everything into Mongo and called it a day.

Or they had someone on the team who insisted that Nosql was the way to go. They'll say something like, "I worked on xyz team at Amazon and we just used DocumentDB for everything and never had a problem. Those horror stories about Mongo are all from people who didn't use it right."