It first started as a joke.
I needed a second pegasus character for the fourth PC (Revision 1) comic to make a shipping joke. Initially, I was gonna go with Featherweight, since “Ponyville Confidential” just aired. But Rumble from the previous episode stuck out more for me.
Really, I wasn’t supposed to bring him to main character status but then I started writing in more of him and then shipping-mentality (and the Hospital arc) happened.
And yes, the fact that I’m one of the first Rumbleloo shippers is bit of a bragging right I gave myself. According to DeviantArt, I am the first person to draw them shipped together, predating the second person by 3 days.