By "siding with Iran" the Saudis mean not condemning the Iranian government after Saudi Arabia executed a shiite cleric for no good reason Iranian people trashed the Saudi embassy over it, and the Iranian government moved to stop the trashing but didn't act as harshly as the Saudis wanted.
Because Lebanon didn't jump on the Iran-condemnation bandwagon, Saudi Arabia then canceled $4 billion of aid to Lebanon, and warned its citizens not to travel to Lebanon. The governments of Bahrain, Kuwait, and Qatar also warned their citizens to stay away from Lebanon and the UAE banned its citizens from traveling there and cut off diplomatic relations with Lebanon.
I expect the moves will devastate Lebanon's tourism industry, which normally gets a lot of visitors from the Gulf states. All because Lebanon did not condemn Iran for merely arresting anti-Saudi protesters, instead of summarily executing them or something.