Because Bitwig has been set to receive stop/start/continue midi messages. You will need to consult your Bitwig manual to see how to disable this, or how to set Bitwig to ignore the messages.
Not sure about slaved, but when the Beatstep Pro is master, it will send out Midi clock info; you can't turn this off. It also sends out a midi CC for start, stop and record. You can change these parameters, but again, you can't turn them off. I've set my transport buttons to send CC124, CC125 and CC126. These don't mess with my synths, but I did have a short period before deciding on this range of some synth's patches resetting (CC123) and other odd behaviour due to the upper CC's beyond CC120 controlling some global midi settings that control the overall behaviour of the instrument (depending if the makers used them). Consult the midi implementation charts for your various instruments to find out what CCs they use beyond CC120. I say use those upper CCs as generally you will find that makers don't use all of them, but will squeeze as many patch parameters they can into spare CC's below CC120. Or at least Waldorf do.....