This is in no way a bug but the expected behaviour and the result of
the new multi-threaded image decoder implementation, that allows to rez textures (much) faster.
The viewer simply makes the best use it can from your CPU by saturating its cores when it needs to decode textures.
If for whatever reason you do not want your CPU exploited in the best possible way, you still can manually configure the viewer to reduce the maximum number of cores in use during image decoding, or even to revert it to the older releases and other viewer's behaviour (mono-threaded image decoding), by changing the NumImageDecodeThreads debug setting (set it to 1 for mono-threaded, or any other number for more threads; the 0 default will let the viewer decide how many threads your CPU can run)...
There are also the "Duration of texture fetch boost after TP" setting (in "Preferences" floater, "Cool features" tab, "Miscellaneous" sub-tab) and the "Boost textures fetches with speed" (in the "Advanced" -> "Rendering" -> "Textures" menu) option/feature that you may want to disable if you prefer a cooler CPU at the cost of slower textures rezzing...