If you break the bottom of a kelp plant, the very top has a chance to grow taller. This is rare and can be hard to see, however it does happen. This can break kelp farms due to kelp creating a water source, if it grows into flowing water.

Attached you can find a video of this happening, all plants in this demo are already 15 tall (bedrock max) and should not be able to grow taller. Once you break the bottom, the tops have a chance to grow taller.