Why? Because calving means the breaking-off of a small iceberg from a larger icemass, with that chunk being referred to as the “calf”. This also comes back to cows. Apparently, “calving” originally refers to the birthing process of the cow to produce a calf. The usage of both words regarding icebergs are related.
Greenland produces a ton of calves every year:
Many glaciers terminate at oceans or freshwater lakes which results naturally with the calving of large numbers of icebergs. Calving of Greenland’s glaciers produce 12,000 to 15,000 icebergs each year alone.