These items fade away 20 seconds after dropping, instead of the usual 3 minutes.
That means ranged, mobile, or large AoE attackers probably lose out on most of their coins and cases.
And people who rely on their pet looting "abandoned" drops (like afk'ers in SDH) won't get any coins/cases at all, because items only become available for other people's pets to loot at about 2:45 minutes after they drop.
No idea why this happens. It's only since the last patch (Pink Bean), and does not appear in the patch notes. Could be a bug, but I can't imagine why it would only affect event items. (Other untradeables, such as Monster Park ticket pieces, still last the full 3 minutes).