Why is the Doctor afraid of the Squire?

Dr. Livesey is afraid of the Squire because he has a big mouth.... he cannot keep a secret.

"Trelawney," said the doctor, "I'll go with you; and I'll go bail for it, so will Jim, and be a credit to the undertaking. There's only one man I'm afraid of."
"And who's that?" cried the squire. "Name the dog, sir!"
"You," replied the doctor; "for you cannot hold your tongue. We are not the only men who know of this paper. These fellows who attacked the inn tonight-- bold, desperate blades, for sure--and the rest who stayed aboard that lugger, and more, I dare say, not far off, are, one and all, through thick and thin, bound that they'll get that money. We must none of us go alone till we get to sea. Jim and I shall stick together in the meanwhile; you'll take Joyce and Hunter when you ride to Bristol, and from first to last, not one of us must breathe a word of what we've found."