Here is how this works...
The Jays figure that the talent is about equal for all of their fringe position players (Zeke, Upton, Goins, Coghlan, others). This could include external players like Pagan.
Of the players they could potentially cut, they think Upton probably is the one that might have some trade value. He does have the loudest tools, and if you squint (and ignore recent history) he's a solid 4th OF on paper.
So, they tell teams that they are listening on him and/or they let it leak to an established reporter that he is on the bubble.
Depending on what they can get, trading Upton might be better than keeping him and cutting another player.