any idea why compressor would engage when I turn my heater on?

1998 XJ HELP Thanks

 

Its normal as the ac helps defrost

 

and helps keep the ac compressor lubed during winter.
but the compressor only kicks in and out when you choose either defrost or any ac selection. regular in cab heat like floor or vents won't use the ac compressor.

 

Thanks, yes I've had it set on heat/defrost setting, now I get it