I thought it had something to do with oil/sweat transfer to the plants that resulted in damaged to the pistils, but I could imagine mechanical stress causing damage too. It's anecdotal, but I had two grows where the pistils turned brown within a day or two of doing some heavy manipulation to better utilize my screen. Initially I thought it might have been pollination from a rogue banana or an issue with my nutrients, but they started to recover and put out new hairs, I didn't find any seeds either. I found some reference searching through the forums to pistil damaged due to handling the plants and it was the only thing that seemed consistent with my grow. I haven't seen this effect since I started being mindful of how I handled my plants. I can't say this is the cause of the OP's issue but it's something to consider.