Fergus McMenemie wrote:

Can anybody explain how this happens given that aluminum overhead lines
are constantly exposed to mosture.

Aluminium is a very reactive metal. In open air, it forms a
closely bound oxide layer, which protects it from further
oxidation despite it's reactivity.

In soil, this layer is under constant attack from acidic soil
water, and is constantly eroded. It doesn't last long once the
insulation has gone.

--
Grunff