Hezbollah works out of Lebanon. My understanding is that these missiles were on their way there, to be used against Northern Israel. Syria is an Iranian proxy, the shipment was likely en route, on orders from Tehran. These arms were not destined to support Assad, and their destruction was not intended to set back the Syrian rebels - simply a preemptive strike to prevent launches in Lebanon from targeting Israel. Thank you.
I see the logic. I suppose its possible that Israel is in Syria posing as anti-Assad forces, gathering intelligence and undermining the radicals as well.