Being a Linksys router someone might have pushed the SES (SecureEasySetup) button on the front panel which invokes an automatic security configuration wizard.
Otherwise if you did not have security set up previously or used WEP then you have probably encountered a wardriver, possibly trying to be helpful in this case as WPA/AES is the most secure form of encryption for home use. If that were the case you should consider it valuable lesson learned!