I've spent a large amount of time fending of bitcoin miners from abusing my site https://cloud.sagemath.com. I like the mathematics and promise of bitcoin, but in practice it is an enormous waste of resources (time, electricity, etc.).
Probably from people wanting to take advantage of free computing power. From their website: Write, compile, and run code in most programming languages ... is a free service ... currently 288 cores, 1.2TB RAM