The explosives belt she had on, was linked to a cell phone, which is often used as a detonator in such terrorist attacks by a co-conspirator who sends a text message when the suicide bomber gets near the victims. What the group didn't plan for, was a random text message sent to the Russian carrier's subscribers, wishing them a Happy New Year.
This reportedly triggered the explosion in the safe house she was in, just hours before the woman could get to the crowded Red Square, where hundreds of thousands of Russians usually gather to ring in the new year. It might have been an even worse blast than the one in Domodedovo, considering how tightly packed people are at these events - think Times Square when the ball drops. Next time we get a carrier's unsolicited message, we won't be fuming with annoyance.
source: DailyTelegraph via Textually