It is not clear yet what causes this strange action. Simply telling it to charge the phone is enough to ring the emergency services. For some unknown reason it thinks charge 100 means charge 999 and calls that number giving the user 5 seconds to end it. It is a possibility because 100 is the first 3 digit number that triggers an odd action in Siri to ring them.
In a lot of cases when Siri makes a mistake the funny side is often seen for example when it plays a completely different song to what you asked for however given that emergency services are involved it is a lot more serious and needs to be resolved quickly as it could potentially prevent someone form’s life being saved.
Some teenagers have been seen to be doing this on purpose as a prank on twitter and although obviously stupid have yet to be punished. Apple insists that they didn’t plan on having this feature on site meaning a possible bug on the phone.