The answer to Is the neutral wire considered safe? states:
When everything is working correctly, it should be at most a few volts from ground. However, and this is the big gotcha, if there is a break in the neutral line between where you are and where it is connected back to ground, it can be driven to the full line voltage. Basically in that case you are connected to the hot line via any appliances that happen to be on in that part of the circuit. Those can easily pass the few mA it takes to kill you.
And something similar is stated in an answer to this question:
A second problem with connecting the ground to the neutral happens if your neutral wire breaks between the outlet and your service entrance. If the neutral breaks, then plugged in devices will cause the neutral to approach the "hot" voltage. Given a ground to neutral connection, this will cause the chassis of your device to be at the "hot" voltage, which is very dangerous.
Can someone shed light on this behavior? Specifically, how a break/disconnect in the neutral wire can suddenly cause it to have full voltage?