The mechanism for this goes beyond the scope of Step 1, so you don't really need to know why we see this. There's a lot of mechanisms involved at different parts of the time intervals.
Note that shortening of the QT interval is mostly in regards to Digoxin toxicity. You won't really observe this at therapeutic dosages. Anyways, early on, you'll see QT interval prolongation since the Na+/K+ pump is an electrogenic repolarizing current, so blocking it increases the action potential duration acutely. Digoxin's effects on the Vagus nerve also lengthen the action potential duration and the QT interval.
But, as we know, Na+/K+ pump inhibition increases the intracellular concentration of Na+, leading to increased Ca2+ concentration. This slows the accumulation of Na+, which affects the Na+/K+ pump, and shortens the action potential and the QT interval. Also, because the intracellular Ca2+ concentration increases, there will be modulation of Calcium-dependent currents, increasing activation of the delayed-rectifier K+ currents (a repolarizing current) and inactivation of the Calcium channel (a depolarizing current), which also contributes to the shorter action potential and QT interval.
