As with her speech itself, the tell is that the decision was apparently so rushed and sudden that there was not enough time to come up with a plausible cover story or to get out the word about what it was.
It looks like a duck and quacks like a duck. Either Palin is resigning ahead of some titanic scandal (which should emerge in short order if it exists) or her resignation was triggered by an even more extreme mental instability than we’d previously suspected.
I think Josh Marshall at TPM nails it with this. My bet’s on the scandal option.