Depending on age, it could just be natural progression. Pistils die and the turn a rusty color etc. This does happen normally. Usually see some plumping coinciding at this stage.
If it's 'too early' it's a sign of pollination. Anything less than ~30 days i'd be a bit concerned about non-white pistils. Once pollinated, pistils die and turn a rusty color. Consider that threshold a wide grey area. I tried to pick a number where it would definitely be pollination vs a fast developing plant.
Wind burn can do it, too. If a fan is directly blasting it, it will kill the pistils and they will turn a rusty color.
Too much light will turn pistils into yellow nubs, so that looks different and can be ruled out. You have 3 options to rule out.... well 2 of them and what remains is the best answer.