Why does the A-4 Skyhawk sit nose-up when on ground?
This was pretty common on naval aircraft from that era which used the launch bridle catapult interface. It gave the aircraft a high angle of attack during launch and helped the aircraft get clear of the bridle at the end of the launch stroke. It's noticeable on a wide variety of aircraft, including the F7U-3, F4D, A-5, F9F, F8, Super Etendard, Buccaneer, F3H, just to name a few.

Even the F-4 did. The nose landing gear oleo strut could be extended to give a nose high squat for cat shots:


(theaviationgeekclub.com)