One of the import options in the import dialog is to "Copy as DNG". There are several options at the top of the import dialog. It seems that some users are experiencing that as a selected option even if it isn't one that they would have chosen. When you are importing images check to make sure you have selected Copy rather than Copy as DNG, and then Lightroom will not convert the images. I don't know if this is what happened in your case, but it is one possibility.
