With leverage you can long or short more than the available supply of tethers. You are effectively creating fake tether with leverage, and then selling it (or buying it). This stuff also happens with equities, commodities, etc.

If there was no leverage, then yes, a 1:1 backed stablecoin would hold it's value.

EDIT: I mean the story here is far from clear, you also need to consider arbitrage bots that act between exchanges, etc. etc.