Funny you should mention that…

A prominent faction in the Roman Republic, and one famous member in particular (Cato the Elder), had been calling for war for quite some time at that point, mostly out of chest-beating ultranationalist bloodthirst. They failed to get a majority to support their cause… until Carthage, on a technicality, violated the terms of their last peace agreement.

For Romans, violation of a peace treaty was casus belli enough, and so started the Third Punic War.

Worst part is, the violation was that Carthage was not allowed to raise any army without Roman permission, a term agreed to after the vicious and brutal Second Punic War some 60 years before.

… but eventually, Roman allies in North Africa began to use that treaty term to harass Carthage with impunity, since they would only have to deal with local militias - and Rome would always take the side of its allies when there was any ‘ambiguity’, in ‘mediating’ such situations. Carthage got tired of it, raised an army… and lost the ensuing battle against Numidian raiders in Carthaginian territory.

So the cause for the war was that Carthage had illegally defended itself.

Romans were very skilled about crafting treaties that could be interpreted like that. Ignored when convenient, because “common sense dictates that it’s not a violation (but we would never outright say that and give you ammunition for a precedent)”, activated when convenient, because “It’s literally the treaty you signed, in plain terms, what else did you THINK it could possibly mean!?”