Firstly check the voltage of output pin in nrf51 without connecting transistor circuit if it's voltage level changing according to setting pin High & Low. If nrf51 reset then it's firmware error.
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.
Your problem look like similar: link