I used to strongly defend that it was in fact some sort of 'emergency' egg laid by the queen. There are a few reasons for this, it seems to fit in nicely with the general survival instincts and characteristics of the species, also in A3 the unique 'queen' facehugger that the egg bore (which the egg itself was unique with the five petals rather than four)
However, the placement of the egg doesnt seem to be anywhere where the queen was, certainly not in the landing gear of the dropship. Also, the queen was never out of site while on the Sulaco (except when in the landing gear), so its highly unlikely that she somehow managed to lay this egg unnoticed.
This leaves Bishop. Bishop isn't like Ash, hes a better modle, but that doesnt mean he isnt suseptable to being controlled or bound by commands given to him by whomever. He had the time, was in the right place, and could be controled (I believe it may have been something along the lines of someone at WY patching in to him or programing him to not remember doing it so he could overide the rules about harming humans). I do believe the egg was a special egg with the queen facehugger for the purpose of keeping the species going or spreading, but I don't see enough evidence to support it was laid on the sulaco by the queen.