General Disclaimer of Everything: I own nothing.
Elementary, my dear Baggins. Or: The Dragon wasn't quite what they were expecting.
"Tell Thorin that I'm willing to negotiate, but that I won't tolerate thieves." Smaug said.
Bilbo froze. Despite his invisibility, the dragon was looking right at him. "What?" He asked incredulously.
"I said, tell Thorin that I'm willing to negotiate, but that I won't tolerate thieves." Smaug repeated.
Bilbo stammered for a moment. Then he managed to ask,"But how?"
"It's quite simple, my dear fellow." Smaug told Bilbo kindly. "Judging by the skid patterns of the coins, the length of your stride falls just short of that of the average dwarf's and your height is also similar to theirs. You didn't think I leave all this treasure lying around because I like looking at it, did you? Now, you smell of dwarves and ponies, but you, yourself are neither. Therefore, you must have spent several weeks on the road with dwarves at the least. They must have hired you for something. Mining has been steady, metal work has not suffered, and the crops were good last year. The only dwarves desperate enough to risk a confrontation with a dragon are the selfsame dwarves that lost this mountain and were displaced from their home. I believe the oldest surviving member of their Royal family is named Thorin. Naturally he would lead any quest to reclaim his homeland. Setting that aside for the moment, the only naturally invisible creature in middle earth is the Urisk, better known as the Brownie. As you have not come to offer me tea, you are not one of them. Additionally, there are only five wizards in existence and you do not smell like any of them. Therefore you must be carrying a magical object that makes you invisible. This means that you are either an incredibly wealthy being, a lucky adventurer, or a skilled thief. Wealthy beings have money. Why poke a sleeping dragon when you can pay someone else to do it? Isn't that why the dwarves hired you in the first place? So you are not wealthy. Adventurers have at least minimal weapons training and they do a great deal of running in places with uncertain footing. However, your own efforts were clumsy and uncertain. Running is not a skill of yours. My conclusion is then that you must be a burglar hired by Thorin. Am I wrong?"
Bilbo trembled. "Truly, you have dizzying intellect, O Smaug the terrible!"
"Run along and pass on my message then," Smaug said with a smug smile. "I'm willing to negotiate, but I won't tolerate thieves."