A dirty little secret within the world of clandestine operations is that the majority of official-status officers (diplomatic or military cover) are known by the host country intelligence services. This is largely because you generally only take so much risk with ongoing collection efforts, for example going through customs and attending official diplomatic functions (dinners etc...) under status is typical and gives the locals a record. What they do in their "off time" is the real collection work, but even then their chance of gaining access to well placed contacts that have very specialized intelligence is low. The majority of officers know this, and it ends up being a political dance, where if someone gets too risky with their collection they get tagged and persona-non-grata'd.
The real question here is why the Soviets decided at that time to burn the officers that it knew about - because that turns into a tit-for-tat across waters that eats up resources and really strains diplomatic relations. I think the answer is very clear in that it was the last gasp of a dying system, so they were doing what they could to purge and wrest power.
Nothing particularly shocking or complicated here if you know how the system works.