Ok, I asked my friend. She works in finances and investments.
After a disaster like in Japan, the local currency gets strong because there is high demand for it after the disaster. Supply/Demand.
The local currency is needed to help rebuild, pay out insurance claims, keep companies floating financially until they can return to business as usual.
Since there is a spike in currency demand after a disaster, the price or power of the currency goes up. Basic supply/demand stuff. So ok, I understand it now.