Maybe the bus was running late and he was trying to shut down the timer, maybe he was 'just' trying to answer the phone...
Not to be funny at the expense of victims, but I can just picture the face of the guy fumbling in the satchel...bet he looked just like 'wily coyote' whey he realized that he was 'had'. But, he only was disappointed for a moment.
Maybe he had just found the bomb, which he previously did not know was in the bag, AND was then trying to shut off the timer.
I don't know though...they said the guys all came, with their "rucksacks" into the area at the same time. It's on video. Bomb, or not, they had planned some sort of violence.