Your eye muscles maybe just tired from constant staring, or neck tension causes brain blood flow problems or something. I struggled with this all the time, but supplementation and exercise helps a lot.
Technical papers aren't THAT boring, after all!
For me it is if in a cosy position and doing something cognitively demanding. Fell asleep very often during my university studies reading literature. It can happen also if trying to learn something new technical at work, but only if I don't sit at a desk.
If normal or not I can't say...
No, they aren't boring at all. I love reading them. And I don't fall asleep reading other things, like news articles or fiction. But my brain's response to seeing Greek symbols is to fall asleep.