Agressive Angel

[neon]

I have 4 Angelfish, 2 Koi and 2 Black.  The 2 Koi I've had since they were about the size of quarters, now they are about 4 inches across, the 2 black I got after and are smaller.  They've all gotten along peacefully, along with the other fish in the tank, tetras, etc.  but just lately I've notice one of the koi is becoming agressive and chasing the other angels and other fish.  I've watched to see if I can tell if it's paired with another, but I can't tell.  What could this behavior mean?