Assuming no irregularities in the performance of the reflow oven, the reason that immediately comes to mind is that the solder paste was not properly dried before being subjected to the elevated temperature of reflow.
There are volatile ingredients in the non metallic portion of the paste - like alcohol -and if it is still "wet" when it reaches a zone at about 100Cit rapidly boils off generating copious amounts of gas -like a mini explosion - which could easily displace a component.
The presence of humidity in the paste or the board would have the same effect. I would expect that if this were the cause the same phenomenon would be noticed on many of the small components.
However, It is difficult to believe that the reflow oven's thermal profile was not set up with a suitable ramp up to cause the solder paste to dry out correctly. It would be worth while checking the reflow oven profile to make sure it is correct. Baking of the boards prior to pasting and assembly should resolve any board humidity problems.
Another possibility, that may be more likely, is that the assembly was allowed to stand at ambient temperature after component placement and before being passed into reflow.
If this is the case, it is not uncommon for the surface of the solder paste to dry out creating a skin which encapsulates the volatile material and prevents it evaporating during the slow ramp up. Here again, when it arrives at a high enough temperature it explodes with the same result!
Harold Hyman has been involved in metallurgical aspects of the electronics industry since the 1950's, and in semiconductor development and engineering for STL, Ediswan & RCA. He later joined HTC, a pioneer of vapor phase soldering and continued industry experience Dynapert, GenRad, Teradyne, SRT and VJ Electronics.
From what you describe, it sound like too rapid a temperature increase is causing the fluxes to boil off and eject the component from the PCB.
Change your reflow profile or the solution could be as simple as rotating the PCB 180 degrees before it goes into the oven.
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Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.