I think exaggero is a denominative, derived from the noun agger 'pile' (itself a nominalisation of aggero i.e. "what is brought"). As such it took the default (

I think exaggero is a denominative, derived from the noun agger 'pile' (itself a nominalisation of aggero i.e. "what is brought"). As such it took the default (productive) first conjugation. You can see a similar effect in English denominative <to highlight> which takes the productive past tense suffix -ed in highlighted, not *highlit.