Hi Georgia,
We had this exact same question from one of our managers only last week. Digging into the call analytics I found that there were two types of rejects we were witnessing.
The first were all outbound calls. The agent attempted to contact a customer and cancelled the call, resulting in the rejection. The 2nd were in fact inbound rejections, but they were usually very short in time. I believe that they are related to a headset audio issue, where the call is answered and then immediately rejected. You can take a look at the journey map and see how quickly the call was rejected - in the cases I've looked it is in fractions of a second - far quicker than an agent could react and reject the call.
On further review of our outbound rejections, a large number of those are also headset issues in my opinion. If someone tries to make an outbound call with their headset audio settings not working as they should (VCC site data cookies is a regular issue here) then an immediate outbound rejection will occur.
A couple of ideas worth considering, and I hope it helps.