What can cause a MAC Address to change?

It looks like Amazon don't allow you to choose a specific MAC address but instead allocate you one on machine creation.

Based on https://forums.aws.amazon.com/thread.jspa?messageID=107448&

Nope, joining to a new network won't give you a new address.
Installing a new device, won't change the existing MAC address on your other card.
It'll, it is a single 'emulated' network device.

The only thing that can more likely to change your address is a driver update which happened to me many times during the past. When I have an older card, and the manufacturer got sold, or someone bought the company ... something like that, THAT part of the MAC address will change when I install a new driver. http://en.wikipedia.org/wiki/MAC_address (The first 3 bytes.)