I've never been taken off a flight until right before the doors close.

Example 1 (mid April): Was flying LAX-DFW-XXX in J, with the LAX-DFW leaving at 6am, and instead decided to standby for a much later route. Despite no showing for LAX-DFW, I retained my seat on the original DFW-XXX until T10ish as I was still checked-in.
Example 2 (a few days ago): Was flying PHL-CLT-XXX in J. PHL-CLT was way late and I only had 20min for the connection. Got to the gate at around T15, boarded CLT-XXX no problem. Standby list for CLT-XXX was at least 30 people deep.

If you were indeed checked-in for your connection and had a boarding pass, I can only assume shenanigans or big problems with AA IT. Write a 3ish sentence version of your original post and send to AA.