Sump is sealed for radon remediation and doesn’t drain water. What can I do for water drainage

What is probably going on is that your neighbors' weeping tile systems are draining into their sumps, which are then pumped out because they're below grade and there isn't a storm sewer at a lower grade that can be drained into. The weeping tile exists to direct water that soaks into the ground away from your house, instead of letting it collect against your foundation walls and leach through, or worse, freeze and heave against your house in the winter. Depending on your soil type, rainwater can stick around for months in the soil, and you may also have an aquifer or water table nearby that provides a relatively steady source of groundwater into the soil.

So, what I'm saying is that depending on how much rain you've had recently (as in the past few months) it's perfectly normal for your neighbors' sumps to still be filling. Depending on the land grade, it may also be normal that your sump isn't yet filling while your neighbors' are (water will soak more into land that is downhill). Just make sure your sump pump is in good working order and don't worry about it any more, unless you start to see damp areas along the walls or floor of your basement; these would generally indicate blocked, crushed or missing weeping tile.

You removed the 1 way check valve, likely part of the pump, when you cut the line to the pump.

Your sump being low, already had a syphon ready to go, it just needed one last push to get started.

The pump of one of your neighbors started and everything in the pipes of all 4 pipes came flowing into your sump.

Your son witnessed an excellent lesson in gravity and syphons. Far more impressive than anything he will be shown even in college physics.