the use of grub in this sense predates "grub stake" by about 200 years and probably (reading between the lines here) stems from the use of the verb grub as transferred Of animals: To root, search for something in the earth, etc.

and the citation: 1659 Anc. Poems, Ball., etc. (Percy Soc.) 22 Let's joyne together; I'le pass my word this night Shall yield us grub, before the morning light.