What is causing the brown leaves?

What kind of plant is it? It looks like too much sun or heat stressed.

If you are experiencing the high heat and hot winds that we are here in the midwest, it's probably being burned up. Set it in the shade with other plants that will shield it from the wind and see if it revives.

Cut off the long tall stem. Put the plant in a slightly deeper pot, with charcoal or rocks in the bottom, and more potting soil. Only add a pinch or two of MiracleGro every three weeks. Water deeply, but not daily. Move the pot so the plant only gets morning sun. Web, you have a lovely plant. I believe you can save it. ☺️

I always have a shaded corner somewhere that I use as a recovery spot, out of the sun, wind and where it wouldn't get hit by driving rain. I have had good luck with doing that with potted plants that are really stressed out and not healthy. I keep it watered, but not too much, even if I have to give it some each day. I give it a little time before I fertilize it, just in case it may have decided to go dormant while it tries to recover. Once it starts getting better, I use the Miracle Gro at it lowest dosage and give it a little bit each watering for a few days to give it a boost. This is what I do. I hope your plant recovers quickly, it is a beautiful plant!

it is a Mandevilla vine. Yes it may have been sunburned most likely from changing location. (I have mine in full sun, but acclimated it slowly). This link may help you. https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm

Over watering or poor drainage. Pot it in different pot with stones or gravel on the first layer.