Here's how Tim O'Brien, associate director of the University of Manchester's Jodrell Bank Observatory, described the boom and the time gap (we quoted O'Brien in a post earlier today):
This reasonably large chunk of rock was moving faster than the speed of sound, maybe 20,000 miles per hour. It made a sonic boom in the atmosphere, and that hit buildings and shattered windows. That is what seems to have caused the injuries.
It's a completely abnormal experience. This thing appeared in the distance, raced over the horizon and was followed up 30 seconds or a minute later by a huge boom as the shock wave hit the ground. I can imagine that would be very frightening.
In fact the boom happened for some witnesses more than two minutes after the meteor came visible, and smaller pops followed the boom.
The meteor crossed the sky a full 2 minutes and 20 seconds before the explosion, which can be heard at the 22 second mark of this amateur video. The video also captures what seems to be the unidentified popping reported by witnesses afterwards.