Its not always sinister, sometimes its just the petty bureaucracy of HR trying to standardize things to benefit themselves. I once got switched from salary to hourly because HR was trying to align job titles across departments.

My title at the time was Sr. Analyst and they were switching to a numerical title system. My boss said to write up a justification for why I should be slotted as an Analyst 3, which would stay salary, otherwise I would be slotted as an hourly Analyst 2. I asked if there was any benefit to the higher title and was told no, so I declined and was switched to hourly. For me it was fantastic because I regularly worked over 40 hours and traveled frequently, so I started getting lots of overtime pay.