The rated power of an S5 is 590 Watts and you have a 600W power supply. Without any further information, I'm guessing that the miner might want to pull more power during startup, but can't.
Also, going for a bigger power supply can be a more efficient option. As far as I know, the top efficiency is reached (for a PSU) when running at ~80% load. So if e.g. the maximum efficiency of a PSU is 80% at 80% load, it is optimal to operate here. Going above or below with consumption and you lose efficiency.
Check out this thread as well
Antminer S5 power suply