SpamSieve went insane today and marked every iCloud.com message as bad and moved them all to Junk.
Nothing else changed at all in Mail. Nothing.
How do you know it was SpamSieve that did that? If you use the the Open Log command, does it say that SpamSieve predicted all those messages to be spam? SpamSieve only looks at old messages if you manually Apply Rules to them in Mail or if you edit a rule in Mail and click Apply after saving the rule.