Because the original script will pick any interface which isnt a slave, passthrough, loopback or contains the name "bridge" and if such interface is type ether its MAC-address will be duplicated onto the bridge.
Which means its highly likely that it will pick up ether1 which often is the MGMT interface but if such doesnt exist then its the first interface on the switchchip (like on a 8 port CRS112).
This gives that you will now have 2 interfaces with the same MAC-address - the physical ether1 (whoever that is regarding MGMT interface vs regular switch interface) AND the bridge itself.
This "should normally" not be an issue but will be if the bridge AND whatever interface got picked to have its MAC-address duplicated and both are connected to the same VLAN.
For example you connect any of int2-8 of this CRS112 to another switch and to the same switch also connect int1 (which you use for management) and surprise...