Igh,
The more I read about the behaviour of the atmosphere, the more I realise how little we know about what goes on up there. At high altitudes you can actually get ice crystal "showers" as water vapour condenses directly into ice, not cloud, which then descends under gravity until it reaches warmer layers and evaporates. That's more or less what's happening to the fallstreaks you see on typical mares' tails. And probably causes the sort of icing you described.