This is a known bug, it seems -
There are also cases where AI either spawns in water or somewhere across a large-ish channel and try to swim across. Naturally, they often take ages doing so and this in various cases can interfere with the general flow of battle (i.e: no further units spawn to put up a proper fight when half the charlie population is swimming) --
βββββββThis is all quite strange, since as we all very well know: Charlie don't surf!