In this war, when IDF meets little resistence early on, that's prime time to start worrying.
Hezbollah is under orders to hide out and avoid committing contact, then sapper IDF half to death after israel relaxes even one iota.
Marjayoun sits squarely astride the only high quality road out of the Bekaa that exits south of the Litani.
The chances of Hezbollah letting it go without even a whimper are less than zero.
I think Israel is just trying to pacify everything south of the Litani River so that they can accept no excuse from the UN and Lebanese troops if this new de-militarized zone fails in the future.