“A-” preposition 1 strikes again, a reduced form of “on foot” – a phrase still very much in use – and indicating a situation in the middle of.  I can picture a member of the infantry from any century in human history, just being in the middle of the foot situation, no way out but through.

• 06.069 to know what was afoot;
• 07.122 there was more wickedness than this afoot,
• 15.002 as if a battle were afoot!’

“a, prep.1.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“afoot, adv. and adj.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.