First, note both OED entries are old, from 1888.
Calve, v.2 (the collapsing bank) is equivalent to cave in, where the dirt slides down to fill a void. And the dictionary suggests that is may simply be a Lincolnshire pronunciation of cave in, but it is possible there is a Dutch root. The OED notes in v.2 that the two senses may be the same.
But calve, v.1 is giving birth, producing another. In this sense the iceberg “gives birth” to another berg. A different metaphor is in play.
As for Paradise Lost, it definitely belongs in the “giving birth to a calf” sense. The line is from the creation of the animals, and in context reads:
In Forrest wilde, in Thicket, Brake, or Den;
Among the Trees in Pairs they rose, they walk’d:
The Cattel in the Fields and Meddowes green:
Those rare and solitarie, these in flocks
Pasturing at once, and in broad Herds upsprung.
The grassie Clods now Calv’d, now half appeer’d
The Tawnie Lion, pawing to get free
His hinder parts, then springs as broke from Bonds,
The animals are rising up from the grassy ground. First the cattle. And after the ground has calved, the lion appears.