In the Microsoft Office certification software that my school uses, in the skill review mode, when I try to highlight something by double clicking the text, it makes the text larger.
It also happens on single click (possibly to make changing the text size easy), and you can also highlight by dragging and it'll work, but I still like to double click to highlight.
I often do the double click and drag to select by word like OP does, but double tapping a touch screen is a common gesture to zoom in. I would be annoyed too.
Are you sure your mouse wheel isnt busted? Because clicking and scrolling at the same time will zoom just like that and if your scroll wheel is broken then it might be scrolling a bit each time you click which will cause it to zoom. I would bet on this because you say it happens when you single click sometimes also. Have you tried swapping your mouse and seeing if the issue persists?
Ctrl plus click will zoom like that. I would bet on some sticky keys or a broken mouse/keyboard. I had a trackpad that did something similar when the battery in my laptop started going. The battery became puffy and was making the trackpad think it had two inputs on it instead of one.