If you park in tall dry grass on the
roadside, watch for what? Potential fires that could occur due to your exhaust
system. Dry grass has a good chance of
burning when it comes in contact with a hot source so turn off your engine to
avoid this kind of situation.
So, if you park in such a situation, your car will be surrounded by an easily inflammable material that is potentially touching it in many places. Therefore you should watch out that you don't start a fire, for example if the heat from exhaust system causes the grass to catch fire.
Theories.
After numerous observations, testing, etc., it is considered a theory, but is not absolute because new scientific can prove it wrong.