It's a strong feature of Attention Deficit Disorder as I understand it. Hyperfocus often comes hand in hand with extreme fatigue and brain fog in the presence of tasks that don't motivate enough, to the point that you can feel it physically.

I often get it myself, feeling like I can't physically write another line of code until I move onto one of my own projects, then I'm good to concentrate for another 6 hours. I have to work extremely hard to complete tasks that on the surface look very simple and involve solving no new problems. This also extends into my normal life, like cleaning a messy room.

Note: I haven't been diagnosed with ADD and am not suggesting anyone has it if they experience this, I've just looked into it a lot.