The antenna is a short... that how current can flow in the system and allow the transmitted power to flow.
If you remove the coax connector from the CB and check the resistance from the center pin to the outside of the PL-2590 Connector, it will read shorted.
If you remove the coax from the antenna mount and check between the center conductor and the shield it will read shorted.
The trick is to keep the coax by itself from being shorted.
With an SWR in the 3 or 4 rang, start by checking the coax for a short.
Then make sure you have a good ground from the antenna mount to the frame and double check the shield of the coax (the outside of the PL-259) and the mount.
Check the assembly of the antenna stud mount making sure the fiber washer is in the correct place.
You don't want the center conductor to ground out or short to ground, until the transmit power has gone up the antenna and came down again.