I think it had to be the plane type and color. This happened to me and my bro.
We have red tails around here and we always see them right by where we live. We also see them down at the local field alot where we fly. When we still had our sailplanes...I had a solid yellow plane that was about the size of a hawks wingspan...and my bro and the same plane but it was white with some black. Now, when we did the sailing part of flying with no motor you could mistake it for a bird pretty easily just catching some updrafts. One day I flew mine no problems. I saw a pair of hawks but it was fine. My bro flew up and after it was sailing for a few minutes...it just got wrecked. It was kinda hilarious since they weren't expesnive planes...but I was surpirsed that mine was left unharmed. Try painting your plane some color that red tails find unrealistic or non-threatening...but I honestly have no idea what color. Maybe just regardless the hawk will attack any threat whether it is real-looking or not...