I tested it again, this time purposefully stalling the 5v cooling fan and there was no shutdown.... oh, but then I realised the cooling fan is brushless. I tried a brushed motor of similar size to the servo motor and, yeah. You are entirely correct. Well, this has been a learning experience. 
I'll have to source a smaller servo I guess. I'm stuck with the 1amp supply. Thanks for your help. I can't immediately solve it, but at least you've stopped me pulling my hair out about the unknown error.