From what I understand Rome and Carthage had been allies against Pyrrus, so why exactly did they suddenly become enemies when messana asked them both for help against Syracuse ?
Because they wanted to be sure someone helped them. Both wanted to help to increase their power. The reason it led to a conflict was because of an agreement they made. It said that Rome shouldnt expand to Sicily and Carthage shouldnt expand to Rome. The problem was that Messana lies in Sicily. Carthage thought this was against the treaty, and tried to show Rome their power with things such as attacking Roman ships going to Sicily.
We cant say which of them caused the war. Fact is that both of them did absolutely nothing to evade a war.