Ok this is just weird... could use some help here!

I have a Pentair Challenger - High Efficiency Pump 345206 1.5hp

It was leaking water from around the center, so replaced the shaft seal.
In the process, we broke the impeller (trying to get it off) and replaced it with the identical model number (355315)
(verified the part is the same- both have the same number stamped on the impeller)

After installing it everything looked fine, no leaks, no weirdness
Pump turns on, pressure goes up to around 14psi on the sand filter and runs.

The next morning, on startup, the pressure climbs to around 25psi for the first second, then goes back down to normal 14psi...
both the pump and the sand filter EXPAND dramatically! and water blows out from around the strap and the sand filter seal.

short video of pump on startup:

View of sand filter & pressure gauge on startup:

I took the pump apart again, removed impeller, looking for problems. Could not find anything weird.
Re-assembled it, it works fine. Just goes up to 14psi, no water spraying out. Normal.
I thought I had "fixed it."

The next morning, it's doing the same pressure building high and spraying water from around the seal again!

I'm deeply concerned, especially seeing we are going on vacation tomorrow.
It so looks like it's on the verge of exploding!

Not sure what else to do?
HELP! Suggestions?


40k gallon in-ground pool, sand filter