[SunkenCiv]

One large impact event (comprised of many impacting objects or ejecta (or both) may have had something to do with it.

[Fred Nerks]

this is interesting:

Recent evolution of water-ice at the south pole of Mars

About 10 000 years ago the precession cycle was inverted, and started to return to its current configuration. Water-ice at the South Pole became unstable, and was forced to progressively return back to the North.

reminds me of something going on 'down here'