I’m sure this bug has been around for years, possibly back as far as Outlook 98: When reading an email, Ctrl-R is the shortcut for Reply. When writing it, it’s a shortcut to right justify the current paragraph. Even when you’re writing a plain text format mail which has no right justify.
If it was using the same shortcut keys as Word, you could left align it with Ctrl-L, or centre it with Ctrl-E. But neither of these seem to work. Not that it matters greatly, since being plain text, it loses the alignment during transmission.