Yes, this is a common spammer ruse.   Often, the added text is simply random strings of letters or words that have no intrinsic meaning. In other cases, the text is lifted from novels, news articles, scientific publications, or a range of other textual sources.

Spammers include this random text in their email campaigns as a means of tricking spam filters. Spam filters are often configured – or ‘trained’ over time – to detect messages that contain certain words and phrases that are commonly used in spam messages.

If enough of these indicators are detected, the filter will treat the message as spam and deal with it accordingly.

However, the inclusion of random text on unrelated topics can fool spam filters into treating the message as legitimate and not flagging it as spam.   Thus, rather than being blocked and sent to a spam folder, the spam message may make it to the user’s inbox where it is more likely to be noticed and acted upon.

This spamming technique is sometimes referred to as  Bayesian poisoning.