If the brown pistils are localised in the same area but you can't see more of them around then maybe you has been polinated and that part of the plant is making seeds so check for that ONLY if it's browning in the same area
It could be a lot of things , can be huidity , if a water drop did falled on them last week then they did get brown .
It can be the signal that she's about to builking up more than before , I've seen plants getting 50% of the pistls brown but just after that they started to push more new white pistols and the calyxes were getting bigger in siwz