An obvious scenario for this would be the Romans kicking them out,
By the time the Romans came to Syria the Seleucids were already too weak to hold on to the East.
The problem was that the core of the Seleucid Empire was in the West in Syria. The Seleucids were ready to sacrifice everything anywhere but to keep Syria. So the Romans came from the most vulnerable side.
what about the Ptolemies doing that
You mean that the Ptolemies somehow managed to push the Seleucids out of Syria to Mesopotamia and further East I guess.
I can imagine that. But I am afraid that the idea to get back to Syria would be idea fix for the Seleucids generation after generation.This fighting would sap their strength and the Seleucids would get totally worned out as a state and get conquered by some Parthians or the like in the end.
But there is another scenario:
The Seleucid Empire might split into the Western Empire and the Eastern Empire when it was strong and powerful.
There were some prerequisites for that division: usually the Seleucid king gave the East to his heir apparent with extremely wide powers.
That would have been quite a natural division and the East would have been able to concentrate on its own challenges - keeping Bactria and Parthia.
So when the Romans come - the Western Seleucid Empire falls as in OTL; but the Eastern Seleucid Empire survives. The Eastern Seleucids might be even bigger than the Parthians of OTL - they might include Bactria and some parts of India.