Polybius makes it sound like their swords had something to do with it. Immediately before mentioning they were drawn up in alternate units, he says:

The shields of the Spaniards and Celts were very similar, but they swords were entirely different, those of the Spaniards thrusting with as deadly effect as they cut, but the Gaulish sword being only able to slash and requiring a long sweep to do so. (3.114)

There is likely another reason though: the Spaniards (Iberians) had fought with Hannibal for several years. They were well-trained and had plenty of experience. It is likely they were led by Carthaginian officers. The Celts (Gauls) on the other hand had joined Hannibal only after he started his invasion of Italy. At the time of Cannae Hannibal had Celtic allies, but it is likely that the Celts who joined him early on were mercenaries (Daly, Cannae, the Experience of Battle in the Second Punic War, p.102). In any case the Celts were led by their own leaders, and had less experience fighting with Hannibal.

By alternating the Spanish and Celtic units, Hannibal put his dependable units next to less dependable units (I'm not saying the Celts were necessarily unreliable, just that Hannibal would have more confidence in the Spanish troops he was more familiar with). The alternating composition makes the battle line as a whole stronger, as a single unit performing badly will have less of an effect on the neighbouring unit.