The complex answer is that a jet engine is a Brayton cycle engine, thtat's what they told us in school, but I don't know what it means.
The last stage of compression, is the point of highest pressure in the engine. The burner can generally opens up in area. This would generally cause the pressure to drop but it only drops a little due to the addition of fuel and combustion. The rear end of the burner section opens even further giving more pressure drop but a large increase in velocity.
If inlet airflow is disrupted, which allows the last stage of compression pressure to drop, the burner pressure will be higher. The air finds it easier to go back out the front of the engine rather than through the turbine section. Another cause could be too high burner pressure, due to too much fuel,
same result, different cause.
The primary thing is that the air finds it easier to go out the front rather than through the engine. The picture shows a combination of increased pressure in the rear of the engine due to thrustreverser deployment and disrupted inlet airflow.
I hope you are now confused at a much higher level.