What Is Doing Scrubby Zoom in Photoshop CC?
This is one of features which you may never realized but in fact it can be very useful. Scrubby Zoom is introduced couple of versions ago and is available from Options bar with Zoom Tool active. Now what is doing this feature? It allows you to quick zoom in and out just by clicking on image and dragging to the right (Zoom In) or to the left (Zoom Out). How fast Photoshop will zoom in or out depends on how fast or slow you are moving your mouse after clicking on image. Once enabled this feature works all the time so you need to know only how to access Zoom Tool from your keyboard: letter Z, Ctrl/Cmd + Spacebar or: with any tool active hold down Z on keyboard + click and drag but do not forget to release mouse button before releasing Z key to go back to previously active tool.
On the other hand if you prefer to click and drag marquee over part of image you want to zoom in, now you know what is preventing you - turn off Scrubby Zoom in Options bar.