A vehicles engine backfiring through the intake is most commonly caused by a fuel to air mixture not being correct which is usually caused by a leaking intake manifold gasket.
When your intake manifold gasket is leaking it can cause too much air to get into the engine which then causes the fuel to mix with too much air that causes the fuel to not ignite properly.
So you then get a backfire through the intake.
Basically the engine gets too much air and less fuel as a result of the intake manifold gasket leaking or you could also have a throttle body gasket leaking allowing too much air into the intake as well.
If you're constantly getting a backfire through the intake you should get the problem fixed because your fuel mileage is suffering as a result and the backfiring could eventually lead to engine damage.