If they had an engine failure at a high power (fuel flow) setting and delayed opening the fuel cross feed, there could have been a rapid fuel imbalance. This is particularly lethal on the Lear 35 with the tip mounted tanks. As the aircraft was enroute to Costa Rica, they were likely full. Excessive fuel imbalance (I believe on the L35 the takeoff tolerance is 200lbs) and relatively low airspeed (Flightaware has them as low as 149kts) could lead to a loss of aileron and rudder effectiveness, and therefore loss of control. Turning left back to the airport certainly didn't help.
All conjecture of course.