If an SMD that is not fixed with glue, but placed in a solderpaste, is soldered, the final position of that component will also be determined by the surface tension of the solder.
This surface tension causes capillary forces between the joining parts that are involved as soon as wetting between the solder and the metal parts, pads and or leads, has taken place.
In the situation that the joint shape is achieved and the solder is still liquid, the component can be "pulled" to its final position when the tensions between all joints involved reach equilibrium.
The capillary forces combined with the cohesive forces in the liquid solder can be so strong that a small component will not fall off even when the PCB is in this situation held in an upside down position.
This explains why the component will normally stay and not move from its original position when the solder remelts on the original joints, as is the case in "double reflow".
The fact that some of the SMD-joints are still molten even after the joints on the leaded components are already solidified can be explained as a result due to differences in thermal behaviour.
Often the heatsink effect of the component body of the leaded component, in combination with the joint layout, is much larger than that of an SMD component. This means that the joint temperature of these leaded joints will drop much faster than the temperature on the SMD joints. As a result of that SMD-joints might still be liquid due to their slower cooling rate, when the leaded joints are already solidified due to their fast cooling rate.
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