No, generally a single ended driver just sinks current. When you turn it off, the current into the driver stops. If there is a decent capacitance in / around the opto LED, it takes a few hundred nanoseconds to actually turn off completely.

The differential line driver (RS485 etc) both sinks and sources current by swapping the polarity over using a totem pole output stage. So generally you have a choice between a sink-only single-ended driver or a full sink-source differential driver.

It's a pity the current solution already uses differential drivers. Do both sides of the driver output still work? Or is the failure due to perhaps one of the lines no longer switching? It's possible that might explain why it bursts into life when the scope grounds one side.