Revenge and loots.
Revenge, because of the merchants were Meccans. There was no war between Meccans and Muslims.
But if they were at war, Muhammad raided civilians, not the military. One of the last raids resulted in the death in death of a guard, and though he didn't kill this guard himself, he was the leader of a bunch armed looters.
But it wasn't war. So Muhammad and his Muslim raiders were nothing more than robbers, brigands or pirates.
And Mecca wasn't at war with Muslims until began these raids, that eventually led to battle of Badr (624). So essentially this wasn't a defensive war or self defence, but aggressive and provocative action. Muhammad is not a peacemaker, but a warmonger.
Loots or plunders were to buy more weapons.
I understand that Muslims who followed Muhammad in exile, lost their properties and wealth. But robbing caravans are wrong, because it is stealing.
Why does Islam say it is a crime or sin, to steal, but it is okay for Muslims to do so, when Muhammad is in charge of robbing merchants? That's not only double-standard, where the rule or law don't seem to apply to him, it also demonstrate to me that Muhammad is a hypocrite.
And in each of these raids, the rule of taking only a fifth of spoil were never enforced, which again, make your prophet a hypocrite.
Stealing is wrong, even if it is done in revenge. Revenge is not justice, it is a selfish act.
Muhammad found a new home in Medina. He could have live out his life in Medina, without robbing merchants, without war. The Meccans would have left him alone. But Muhammad couldn't leave it alone, so Muhammad had initiated this war with those raids.
Sorry, but I couldn't follow a religion where the founder of Islam, go out with his men, robbing people.