It could be cryptomining.
When you are not aware of the mining activity—which is the majority of the time—it is a theft of resources. This is because cryptomining takes advantage of your computer’s Central Processing Unit (CPU) and Graphics Processing Unit (GPU), running it at higher capacities. Imagine revving your car engine or running your air conditioning while driving up a steep hill.

If cryptomining is too prolonged and running at, or near, the maximum of what your computer can handle, it can potentially slow down every other process, shorten the lifespan of your system, or ultimately brick your machine. And obviously, any malevolent threat actors want to keep using as many of your resources for as long as possible.
For more information, see How to protect your computer from malicious cryptomining.