Tillers

I bought a near-new DRT900H tiller from Lowes. The tiller had only been used once by a customer who brought it back because he said "it was under powered." The first time I used it , it did fine for the most part but on one occasion I shifted it into forward with counter rotating tines and the tiller did not move and after a few seconds smoke came out of the drive belt/pulley area. I stopped the engine immediately and tilted the tiller up to inspect the tines--there was no object to obstruct the tines. What is causing that smoke and is the engine damaged? It worked fine at other times when I was in forward gear with counter rotating tines. How do I prevent that from occurring and do I need to worry that the previous owner damaged it?