@InfraBlue,
I think they are generated by "bot" (robot) programs which try to populate Emails with text which won't trigger anti-spam filtering.
Many of these Bots were experimental (and probably buggy; like all programs) and introduced to unsuspecting home systems in the form of viruses which simply churn out spam Emails. Some of these Bots may have been running for years, while their more evolved cousins are probably churning out far more effective Spam.