If I can give my own insight on the matter, this is all assuming there was a battle at Zenan Bridge during the original timeline.
Remember, NPC's state during the time Queen Leene was kidnapped that Magus's army destroyed the bridge. Unless there was some unheard plans of staging a naval assault, that's quite counterproductive to cut-off access to Guardia if you plan to finish it off. So then why destroy the bridge and stage an assault after its repaired instead of just plan the assault in the first place?
My guess? The Mystics/Fiends were riled up since people had all begun talking about the Hero. If you consider that timeline changes due to Crono, Lucca, and Marle's first visit to 600AD made it possible for Tata to aquire the Hero's Medal, and this becoming well spread news, then it would also made an impact among the Mystics/Fiends. Perhaps in the original timeline, with Frog keeping the Hero's Medal and thus no one making a fuzz about a Hero, then the Mystics/Fiends weren't as eager, and figured Magus would just take care of things with whattever he's doing at this castle.
But with the Hero, the Mystics/Fiends now think that perhaps the humans can now do something, and perhaps Magus may not have the time. So perhaps they think it might be best to just crush the humans right there and then. Thus, the assault on Zenan Bridge in the changed timeline.
That's my theory, at least.