Why does Pip describe the seven little Pockets as tumbling up instead of growing up?

The Pocket children are completely unsupervised. Tumbling up rather than being brought up means they're left to their own devices and learning from their mistakes.

And unless I deceive myself on a point where my interests or prepossessions are certainly not concerned, I saw that Mr. and Mrs. Pocket’s children were not growing up or being brought up, but were tumbling up.