why is my boiler firing water out the safety value?

we have a alpha eco240 boiler, my husband topped up system to green area but on coming jome from work the neddle had risen to near red area and the water had beed fired out the safety valve outside the house, can u tell me why my boiler would do this at such a random time, wen the only thing that has been wrong with the boiler lately is the piolet light keeps going out.