It already was said that it is caused by apparel in the loot you already have.
Since there always is a piece of an experimental apparel in the loot of a reaper, it always happens there when taking all of the loot.
For other rivals it may happen, too, if there also is a piece of apparel in the loot.
It was fixed for steam and xbox with the last hotfix and should be fixed for all after the next update.