buck regulator power draw
Your buck regulator is trying to run backwards

Your buck regulator will power up if voltage is applied to its output with its input floating -- this happens through the body diode of the high-side FET inside the buck IC.  The overcurrent is coming from the resulting confusion, I bet -- the controller may try to turn the low-side FET on, which shorts the SW node, and thus your 5V supply, straight to ground via the switcher inductor.  Oops!

You have the right idea with the diodes, just use the right diode for the job

The easy fix is to stick a pair of suitable Schottky rectifiers (a couple of standard 1N5819s will do) in series with both the USB and buck converter power lines -- the ATmega isn't picky about its power supply, and you want to protect the USB power line this way as well because backpowering USB ports has been known to be a motherboard-frying mistake.