Welcome, Don Cannoli. Good question!
Among the possibilities:
--Fredo, at the Presidential palace, skulked off. Michael asks him where he's going and Fredo says, "I'm gonna get me a real drink..." Perhaps Fredo went off to find a phone to call a Cuban official whom he might have met previously, to warn of Michael's plan to kill Roth, and to add urgency because Roth hadn't shown at the palace. In the very next scene, we see the elite-looking military double-time to Roth's hospital room and shoot the bodyguard, so perhaps there's a connection.
--Roth's wife, after leaving the hospital, probably returned to their hotel suite. She would have discovered Johnny Ola's body and, suspecting more foul play, called one of Roth's Cuban government pals to set the military guard into action.
--Batista (the President of Cuba), knowing that his government was about to crumble, in a last act of generousity toward his old friend and associate from Florida, dispatched the military to the hospital to make sure that Roth got out ok.