Are you asking about the bloom or the plant (leaves, canes)?

The blooms turn brown for several reasons. One is an insect called thrips. They suck the moisture out of the petals. If you shake out a bloom, you can see the insects. The other is wet weather. You know if you soak a tissue it gets wet, so do petals eventually if there is a lot of moisture, and if they don't get a chance to dry off, they turn brown and stick together. Probably facilitated by a fungus called botrytis.