water pump WXL

We were away last week in our 2020 WXL and during that time I kept the water tank full but over the last two days I allowed the water level to fall. 

On the last day I guess the tank was still a t least a quarter full but suddenly the pump started to surge and even a slight adjustment of the white pressure regulator knob in both directions didn't make it stop so I just switched the pump off. A little later, whist her the boss was faffing about I switched the pump back on and everything seemed fine, I used both taps on hot and cold and no problem.

My question is, what would have made it begin surge in the first place and if it happens again what should I do?

Thanks for any help and suggestion

12v or 240v ehu, makes for fine adjustment. We adjust pressure switch while OFF ehu, 12v supply from leisure battery.

Thanks for the reply mate we were on EHU all the time.