This could be lots of things. Like many manufacturers, SWR would often work with the manufacturer to produce a custom driver for their speakers. These drivers might closely resemble or be based on a standard model, but with differences in the specs to meet their needs for a particular cabinet. Standard drivers tend to come in 4 and 8 ohm versions, but with a 610 there is no easy way to wire 6 drivers to end up with a 4 ohm load. The replacement drivers may not have the same individual impedance of the original drivers, and that could easily result in more power going to the replacements than the OEM drivers still in place. The replacement drivers could also have different Theil-Small parameters than the original, which would mean that the cabinet tuning could not be optimized for them and the originals at the same time - so the resonant frequency might not be correctly cancelled (resulting in over-excursion like you report.) If this is the Goliath Senior, the manual does state that it is a custom driver. My first move would be to see if any original replacement drivers might be available - check the SWR website for a service location near you.
Sadowsky Club #477