Saudi Arabia said on Monday that Lebanon had declared war against it because of attacks against the Kingdom by the Lebanese Shi'ite group Hezbollah.
Hezbollah gets blamed for supplying Houthis with missiles, like Iran does.
The Lebanese Prime Minister resigned on live TV 'in protest' of Hezbollah's interference with Lebanon's affairs, or something to that effect. The Lebanese Prime Minister was in KSA when he announced this, and Iran, which uses Hezbollah as a proxy, has hinted that KSA is not letting the PM leave, essentially holding him hostage or something like that. That part is doubtful, he is probably staying in KSA because he fears assassination from Iran or Hezbollah. From what I've read, he does not get along with Iran and Hezbollah.
He actually met with the Iranian defense minister very recently and by both accounts had a positive, constructive meeting. He then met with Saudi officials the next day, before abruptly flying back to KSA to announce his resignation. He does not usually get along with the Resistance but many think he was coerced to resign. In addition to the suspicious timeline, his resignation speech was in the Gulf dialect not Lebanese, according to some Arab users here.