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First when you remove water from inside a sealed container you need to replace the volume of water with an equal volume of air or you will collapse the container.
The good news is that we know your pump works. The bad news is your venting system.
When your pump kicks on and begins evacuating the sewage from your container the first place the air has to get into your container is through the properly sized and installed vent connected to the container through a sealed entry. Since there was no replacement air entering the pit or container through a sump vent then the air needed to enter the container system through another path.
The next logical place for replacement air to enter your sewage container or sump would be through your buildings sewer and drain vents. A properly vented sewer and drain system would allow an ample supply of air to enter the system providing all lines in the building are open and working properly. While the forced suction of a pump evacuating a container might cause the water seals in your traps to "bounce", it would not cause an evacuation of a trap if your system is properly installed and designed.
Your container was able to recieve the replacement air it needed to prevent collapse through siphoning your closest connecting fixture empty and then getting the air it needed.
To remedy your problem you need to properly vent your ejector sump pit and then properly vent your plumbing system. Half measures won't do.
A standard sewer pump will probably not have sufficient power to collapse a properly made sewer sump. If you were to fill a sump with water, install the pump and then seal all entries and turn the pump on you will pump out most of the water then probably burn your pump up before the pit collapses.
However, anyone who has ever witnessed a tanker truck have its tank collapse due to the operator forgetting to open the vent valve will completely understand my first post. It is sudden, it is loud and it is ugly.