For the pale in Arms, representeth a post of Timber, set upright, such as be commonly used, to under prop the earth from falling upon the miners’ heads.

– From Lacies Nobilitie by Sir John Ferne (1586), p84

The origins of all of the ordinaries are unclear, but the theory Ferne cites above is a fairly common one about the pale. Unfortunately, it’s likely that we will never be able to know for sure.