Why was Mundungus so loyal to Dumbledore?

Question

From this answer,

“He’s useful,” Sirius muttered. “Knows all the crooks — well, he would, seeing as he’s one himself. But he’s also very loyal to Dumbledore, who helped him out of a tight spot once.

What did Dumbledore do for Mundungus that would cause him to be so loyal, to the point that the Order wants him to help with such an important task as moving Harry in the last book? Sure, Moody didn't trust him completely, but they still thought he was one of the useable people, and while Dumbledore was around he seemed to trust Mundungus as much as he trusted others.

Why was Mundungus unquestionably (or was he?) loyal to Dumbledore?

Answer 1

I think the passage you quoted is all we know about Mundungus’s loyalty to Dumbledore.

It’s not discussed again in the books, I don’t know of any interviews about it, and there’s been no mention of it on Pottermore.

Given Mundungus’s criminal tendencies, I think the most likely explanation is that he was caught or accused of doing something really bad (worse than petty theft), and Dumbledore playing a significant part in exonerating him.

As Dung was following Harry shortly after the Order of the Phoenix was reassembled, I think we can assume that he was probably in the original Order. I would thus guess that he was framed for something nasty during the First Wizarding War, and Dumbledore’s testimony and/or evidence stopped him from going to Azkaban. But that’s only a guess; we know almost nothing.

And as for Moody, remember that he doesn’t trust anybody. I wouldn’t read too much into that.