Threshold of an AP is actually around -55mV. So what is happening is K+ is in higher conc. on the inside of the neuron than the outside. This means K+ wants to flow outside as seen in the descending portion of an Action potential where the membrane potential is driven towards -70/-80mV. By increasing the amount of K+ outside of the cell, you effectively reduce the concentration gradient for K+ to leave the cell. This reduced gradient in turns causes some K+ to "stay inside" compared to before and + charge in the neuron means depolarization. You can also look at it mathematically withe Nernst Equation, specifically the concentration portion of that equation. E=RT/zF*ln([Ion(out)/[Ion(in)]). Normally this value, using physiological values of [K+], is around -5 and -5*RT/zF we get around -80mV which is close to resting membrane potential. When we start to increase the value of the fraction, make the numerator bigger by increasing extracellular concentration of K+ we get a slightly smaller value. When i plugged in ln(15/150) for a hypothetical scenario, we get around -2.7. -2.7*RT/zF yields a more "depolarized" value, closer to 0 but still negative. This effectively will help the neuron fire another AP because we are closer to that threshold potential of -55 which triggers firing. I hope that helped.
