Why there is an Upper Explosive concentration Limit? If we have a mixture of combustible gas (let's say it's methane) and air, what is the physical explanation for having a upper concentration limit? 
I found this document:
Lower and upper explosive limits

For example, methane has 5% and 15%.
Let's say we have a 30% concentration of methane... There is still enough oxygen, so that some of the methane could burn. 
There won't be any combustion at all, or there is some combustion, but no explosion?
In simple terms, an explosion takes place when the reaction front propagates faster than the speed of sound. If it is slower, the expanding gas cloud can push away the unreacted mixture before it reaches ignition temperature.

With too much methane, the combustion reaction slows down, because the additional material takes up energy for heating, but cannot react.

A very simplistic microscopic picture would be that a hot methane molecule too often does not hit a cold oxygen molecule, with wich it could react to produce more heat, but a cold methane molecule, leaving two lukewarm methanes, than won't react any further.