The source imagery can be 8 bit, but if the spatial reference is altered or if the NoData value is specified outside the range of 0-255, then the process will resample the raster. In the resampling, rather than delete data or replace values, the bit depth on the raster will be upgraded to the next bit depth. So if you values run 0-255, a NoData value of say '0' is not set, then the process will make the NoData value 256. Then it will look at the values and upgrade the bit depth. This change from 8 bit to 16 bit usually causes a lot of the loading issues.