In T233729, it was made possible to send a message to reviewer in addition the author of a page AND the new option ( sending to "reviewer") was made the default selection.
The order isn't ideal and have been causing confusion. Many messages are not going where they're supposed to go; see this self-message, this one too as well as this all of which were obviously meant for the "author" but ended up on the "reviewers" talkpage.
A more recent case is here on wiki where another reviewer is also confused.
The solution is simple, the order should be swapped. Sending to "author" should be the default selection