Why does Moon Reader not show up in 8 last apps list?

Normally, when you long-press on HOME button, my Android displays the icons for 8 last apps/activities that were active. This seems to always be pretty reliable, with one exception. For some reason, [Moon+ Reader app](https://play.google.com/store/apps/details?id=com.flyersoft.moonreader&hl=en) that I ran today **does NOT appear on that list of 8 apps. At all.** Is there any possible reason why? *Environment*: Motorola Droid X, rooted stock Froyo 2.2 ROM, Zeam Launcher.