Instead, it turns out, a spam SMS message from her cellular provider accidentally set off the device early. The SMS came in just hours before the planned attack, and killer her, but not her two accomplices, who were seen fleeing the scene.
It is common in Russia for Islamist terrorists to trigger bombs with an SMS message. They use cheap unused mobile phones as detonators, with the bomber's "handler," who is generally watching. The "handler" detonates the device by sending a text message which sets off the explosive belt at the moment the "handler" deems is best for maximal damage and injury.
In general the phones are kept turned off until the very last second. In this case, Russian authorities believe that the terrorists were simply lazy or careless.