Hmm. . .
I think Chy's political bit is likely, but it never struck me as how things were.
Thorin's
dwarves all seem very well off. They're getting fat and rich and having fun.
I can see
Balin at first enjoying the leisure but after awhile the glamor wears off and he starts to look fondly upon
the old days when he lived by the seat of his pants, besides he's getting older and he doesn't want to get older. He has one last quest in him.
The choice of quest is obvious. Dwarves like
Mithril. Dwarves have a lot of pride staked in that old cave. It's bound to be safe by now. That instance with whatever it was was a long time ago.
It is possible he may have been planning it on his way to see
Bilbo, but would he have mentioned it to
Gandalf? I don't think so. Dwarves are naturally secretive. Gandalf may have guessed, of course, but been told directly, no.