The reasons are the same for both powers. Control of the trade in East Indies. The death of the prince just served as an extra incentive for the Spanish king.The justification made by the Spanish king for the death of his favourite son, whom he blamed the Romans for. Both sides in their respective territories in the east have been planning on attacking each other. But the key difference is what instigated it and the justifications around it.
Moreover, as mentioned in the previous update, the Roman's have already planned to attack Malacca, even if the Spanish would have not sent the fleet, which would trigger a major Spanish response. So this response would happen either way. The Spanish simply preemp it