YouTube - Wake turbulence plane crash in Mexico City: Radar View
I think that wake turbulence is a plausable explanation. If I read the data tags correctly, just before MXA1692 (the 767-300) makes the turn to final (2:53 in video), the Lear is at the same altitude, 4 nm in trail and 70kts faster! Then, at the location that XC-VMC has the event, they were 300ft lower than the 763 was at that point and they crossed it 1:24 after the 763.
Speculation, of course, but given the proximity and the altitudes, it fits.