One of the reasons for a traffic drop can also be due to your site losing links. You may be seeing a direct loss of that referral traffic, but there could also be indirect effects. When your site loses inbound links, it tells Google that your site isn't as authoritative anymore, which leads to lower search rankings that in turn lead to traffic drops (because fewer people are finding your site if it's not ranked as highly and more).
What’s the catch? None, really. Cash back apps act as affiliates for many online merchants, which means that whenever you make a purchase through one of the apps, they get a small commission — but then, they give you a portion of that commission as “cash back”. For example, if I buy a pair of Nike shoes through the Ebates app (or website) and spend $75, Ebates may get a $10 commission but then they’ll pass $7 back to me. It’s basically a way to get sale prices on stuff that isn’t on sale!