RS485 implementation using MAX485 chip
The general message is: set all inputs to valid logic levels
A and B inputs are not connected to anything so any state can be assumed on the A and B lines and that will produce either high or low on the RO output (given that you have nRE set low i.e. you are enabling that output). If high is produced on RO then it will turn on the LED. Digging deeper this explains what you see more clearly: -

Try setting A and B to known valid levels.
The RS485 chip knows nothing about data or protocols  - it just acts like a buffer.