Why does my outlet have two white wires on the neutral side and one "neutral" wire is hot?

The breaker for my microwave tripped and when I turned the breaker back on it tripped again after using the microwave again. I tested the microwave in another outlet and it works fine. I went to replace the outlet and I found a black wire, two white on the same side, and ground. I turned off the breaker and tested the wires only to find that one white wire still has 120v where all other wires are dead. Why is that wire supplying voltage on the neutral side?