What does "不要紧" mean?
不要紧 has two meanings (http://www.baike.com/wiki/%E4%B8%8D%E8%A6%81%E7%B4%A7):

Doesn't matter; not a problem.

Seemingly not a problem but ...


Here it is the second meaning. The logic behind the sentence is:

众人回头观望，而这一看不要紧，所有人都是不由大吃一惊
The group looked back. This seemingly trivial action lead to a surprising finding.