Currently, all the holders of tether could ask for dollars, and they'd all get them, and there'd even be 0.26% of the original balance left. (Under quite some assumptions, namely that they could sell the commercial paper at the value at which they hold it in their accounts.)
However, if the value of their assets would shrink, say, by 1%, their equity would be wiped out, and they'd not have enough dollars to satisfy their liabilities. Then, if people would start demanding their dollars, the last 1%-0.26% = 0.74% of tether holders would get nothing, because there'd be nothing left.
That could trigger a good old bank run - you don't want to be among that last percent of bag holders, so better take out your dollars now while they still have some.