Frequent malware in cached pages

Since I started using WP Super Cache, it happens often (almost monthly) that WordFence finds malware (PD9) in cached pages. It’s good news that it’s getting caught, of course, but bad news that it’s there at all. WordFence never found malware on my sites before I installed WP Super Cache. How and why does this happen, and how do I keep it from happening?

Note: I keep WP Core and my plugins and themes religiously up-to-date.

PS: Apparently, tagging with ‘WordFence’ is not permitted?