Check your facts! He is not the CEO of An Post but the Chairman, not exactly a full time job, so it wasn't "bound to happen" as a result of that at all. Personally I think the recent pension-entitlement issue was the tipping point.
CEO or Chairman, once it happened most knew his time at EI was running out. Your may be right about why he is leaving however his own pension package and how it went down with shareholders could be a factor to!