I'm not completely sure myself, but I'll try to break it down based on what I think I know. I'm sorry if it turns out to be wrong, since it's still a bit early to speculate on certain things imo.
Scub Burst is that they call an event where a piece of Scub Coral appears out of nowhere and causes great destruction to the environment. Whenever this occurs, a G-Monster (called Secrets by Generation Bleu) usually follows. There's a connection, almost like the G-Monster is attracted to the Scub Coral in some way.
Beyond that I don't really think we know exactly what happens if a G-Monster is left to its own devices. In ep1-2 it didn't seem like it was going after anything in particular, and just causing tons of random destruction. In fact, after the first Scub Burst, it blew up a bunch of stuff and then just disappeared later, along with the Scub Coral, only for another Burst to happen again the next day.
Seems like there's some sort of unstable dimensional shifting going on if you ask me...
That's fine except for the part where Fleur states that the G-Monster getting to the Scub Coral will cause a huge Scub Burst. So I guess G-Monsters can cause Scub Bursts as well.