I'm not sure even Bill Murry and Dan Aykroyd could help you with this one.
FWIW Dept: When you shut off a TomTom, you're not really shutting it OFF off. Clearly, there's no screen lit and the thing isn't actively keeping track of your location, but the processor isn't actually shut down. It's operating in a low power 'sleep' mode - just barely using any juice, and having nice GPS dreams and keeping time. After a period of one week, the device is supposed to shut ALL the way down (which is why it takes a bit longer to power them up if they've been down for a week or more). In the case of this particular bug, instead of shutting down completely, they did just the opposite, and 'woke up' to an 'on' state.
The main reason for the total shutdown after a week is to avoid completely depleting the lithium ion battery. Even in the super low power 'sleep' mode, it's being drained, and lithium ion does NOT like to exhausted, and especially does not like to be stored in an exhausted state for an extended period of time. Further to that, your ephemeris / QuickGPSFix data (the information about where the satellites will be at some time in the future, giving the unit a leg up on figuring out where it is when you power it up) goes stale after a week anyway, so the unit will necessarily have to start looking for this information from a satellite when it is powered up again anyway.