Only 'push' and 'pop' come from the stack terminology. You 'pop' plates off the top (the rest of the plates 'pop' up when you do it I guess).
I would say 'shift' comes from shell scripting, where you 'shift' the arguments off of the list passed to the script. This is a fairly natural usage, and might also derive from assembly language, where a basic operation is to shift all the bits in a register one place left or right.
'unshift' is probably originates with perl and would have been named by analogy with 'shift' I think.
