I think it is more than just an echo or perhaps an echo in the stadium makes athletes slower to react to the gun because they are hearing other sounds as well?
I went and made a spreadsheet of the reaction times from Day 1 2012 vs Day 1 2010 for the 60, 60m hurdles, and 400s.
Event 2010 2012 Difference
400 Round 1 W 0.309 0.336 0.027
400 Round 1 M 0.245 0.311 0.066
400 Semis W 0.260 0.326 0.066
400 Semis M 0.202 0.226 0.024
400 Day 1 Total 0.258 0.306 0.048
60 Hurdles W 0.187 0.251 0.064
60 Men 0.187 0.268 0.081
The last column is how much slower the reaction times have been in 2012 versus 2010.
As you can easy especially in the short sprints there is a big difference in reaction time with 2012 being slower than 2010. .07 slower in such a short race seems like a lot to this distance runner.
I tried to isolate any differences by lane and couldn't come up with anything significant. If someone wants my excel spreadsheet email me. (And I excluded one person from the data who took .9 to react).