Slitherine

On attached screenshot we can observe a following situation:

Initially zealots (green) were positioned face-to-face with irregulars (yellow). Armored horse archers (blue) also faced the zealots (green), but diagonally. I thought both medium foot ZOC'ed each other out. Next turn the zealots attacked the horse archers, who evaded, and the zealots smashed into the flank of Sasanian heavy infantry (white).

Why was ZOC of irregulars (yellow) ignored by the zealots (green)?

The Zealots were in the primary ZoC of both the Foot and the Horse, and thus had the option to charge either.

And what's the rationale behind this?

I'm confused. I'm in primary ZOC of two enemy units, which is supposed to actually limit my movement options, and indeed it does... but I can still choose to advance towards the unit that is in my own secondary ZOC? Diagonal attack is allowed, but diagonal move into an empty square on the flank of an enemy unit is disallowed. I can maneuver, but at the same time I can't.

You can attack any unit that is in one of your front 3 squares if it is directly facing you. Seems pretty straightforward to me.

Ok, thanks. I guess I mistakenly assumed that my HA are safe and can't be attacked at all in that situation.