It's actually done to prevent clouds of flammable gas, mixed with air, from forming around the vents. The environmental benefit is a plus on the side.
"The explosion occurred after a pipeline began leaking liquid natural gas[1] (mainly propane and butane), creating a highly flammable cloud that was soon ignited by wheel sparks from two passenger trains heading in opposite directions near the site of the leak. "
That doesn't make sense to me. Either it's natural gas, or propane-butane. Where I live at least, there are requirements for what natural gas is supposed to be. There can be some amout of impurities in it but it's still supposed to be mostly methane.
Maybe they meant that the propane and butane impurities stayed low while methane escaped?