what would cause this? do i just need to replace the hose? or is it something more expensive like the water pump, thermostat or a blockage somewhere.

there are two different spots where it has collapsed the there is a lot of vacuum in the cooling system.

 

i believe a low level of antifreeze would cause it,

 

check coolant levels, rad hoses should be stiff though so if its that floppy and loose its time to replace it anyway. 10$ of prevention kicks the **** out of over heating on the side of a road