The Byzantine empire didn't "collapse" in the sense that it ceased to exist until some 800 years after early expansion. Under Byzantine emperor Heraclius, the middle east had been overrun by Sassanid Emperor Khosrau II. Heraclius led a grand campaign to push the Persians out of the Levant. This depleted Byzantine troops and money, something that hadn't been replenished since the Plague of Justinian in the 500's. When the Muslims came out of Arabia, the Byzantines had set up defensive lines in Gaza to keep bandits out but most of their troops were in northern Syria to fight the Persians, allowing Muslims troops to move a vast distance before meeting regular Byzantine troops. The Byzantine army was generally mismanaged (the exact details of which I'm not aware) but when the Byzantines finally fielded a regular army to fight the Muslims, it was defeated at Yarmouk, which crushed the Byzantine Empire's ability to project power beyond Anatolia.

So basically -War with Persia, mismanagement of army