Firstly check the voltage of the output pin of nrf51 without connection other transistor circuit if it's voltage is changing as we set is High & Low and device not reset then it's not firmware error but if you face same error then check your firmware side.
If firmware working fine then check transistor circuit is it properly become High & Low(if not change your transistor). Have you properly connected both GND (nrf51 & transistor).
Check you relay config & circuit (eg working voltage & current level etc)
If all good then lastly change your load.