In this case, the Fix constraint was applied on the sub-assembly(1) which has only one component at the top assembly level. When selecting another sub-assembly(2) to move the fixed component, sub-assembly(1) is also moved unexpectedly. What causes the fix component to move?
Solution There are couple of reasons why the fix component, sub-assembly(1) was moved
1) The customer default: Assemblies --> Positioning --> Interface --> Scope --> Anywhere in Assembly was enabled.
When Anywhere in assembly is enabled, all the constraints in the entire assembly are copied to the display part (behind the scenes) and solved simultaneously.
2) Since the Fix constraint is on a sub-assembly only, which contains one component which is not fixed in the subassembly, therefore, the Anywhere in Assembly mode move is allowed to move the component.
If you want to use "Anywhere in Assembly" mode, this is as designed behavior. If you do not want this move to happen, you should fix the component as well as the subassembly. Or else, do not use "Anywhere in Assembly".
Notes and References
Hardware/Software Configuration Platform: INTL64
OS Version: 764SP1