Sub_Level
wants to fuck an Asian grill.

It's a simple question.
+ Burke never made it back to the Sulaco so it wasn't him.
+ Hicks was out of commission so he didn't have time to look around the dropship while Ripley was busy rescuing Newt.
+ Even though Aliens establishes that Bishop is actually a good robot, it's not like Alien 3 was particularly reverent of Aliens so making Bishop a sneaky, sabotaging robot like Ash has precedence.
+ It also may not have been Bishop's own personal choice if he had programmed parameters that forced him to do such an action but not relay that information to Ripley when he was looking at the flight recorder.
+ There is a small period of time the movie skips between when the dropship lands in the Sulaco hangar and when Ripley and Bishop are talking before he gets stabbed. Even if you don't believe it was Bishop, this period of time and the time when Ripley is getting the power loader ready are literally the only 2 possible time slots the egg could have gotten in the cryo chamber area.