Spring clutter?

Almost two years ago I planted a flat of lovely little pansies in the pots on the back terrace. They cheered me up with their little faces all that fall and then again last spring when they bloomed again, still contained in the pots where I'd planted them. But at some point last summer, they escaped and we returned from our long summer holiday to find they'd sprouted literally everywhere - the gravel in the drive, the cracks between the terrace squares, along the fence. And now it is again spring and they are blooming again, everywhere except the pots.

What happened to "Bloom where you are planted"?