@Thor
It’s not active by default, at least not in my version of the program. However, it is easy to accidentally switch on.
Simply hitting D on a mesh with no subdivision levels will switch it on. On an object with subdivision levels, D/Shift D will move up and down the subd levels, so some people are used to keying that on reflex. Since modern Zbrush now has a number of workflows that involve working on moderately high poly meshes on a single subd level, it’s an easy thing for some people to toggle Dynamic SubD on a high poly mesh which will severely impact performance, and result in the “subdividing polygons” message the op mentioned.
Until they get a chance to maybe take a look at separating the functions from the same key, I recommend switching the traditional D/ Shift D hotkey assignments so that the Shift D combo is the one that moves up subd levels (and therefore switches on Dynamic SubD when no subd levels are present). That way you have to think about it more consciously, and the two button combo isn’t as easily switched on by accident. I have also dragged the Dynamic SubD toggle button to a conspicuous spot on my interface, so I can always see if it’s lit up at a glance.