It’s similar to the the en in this question. The pronoun indexes a prepositional phrase with de indicating the source of the feeling.

It’s an extension of the core/initial meaning of de indicating the point origin of a movement in the direction of the speaker.

“Ça en devient [adjectif]” is something of a set expression (C’en is a more literary variant) meaning “It’s starting to become [adjective] because of this”. You could omit the pronoun without a change in meaning but leaving it in puts more focus on the source of the feeling of boredom as a problem.