When adding edgeloop with loop cut (ctrl+R) the result verts are automatically appended to groups of both other vertices from edge they were created on regardless of vertex group being locked.
When group is locked it grays out assign/remove button and this documentation states:

"When you click on the icon, it changes to a closed lock icon and all vertex group modifications get disabled. You can only rename or delete the group, and unlock it again. No other operations are allowed on locked Vertex Groups, thus all corresponding function buttons become disabled for locked Vertex Groups."

To reproduce:

• create cube
• go to edit-mode
• add edgeloop
• add it to group1
• create another edgeloop over the same axis as first
• add it to group2
• now that each new edgeloop is in its own vertex group, lock the groups
• create new edgeloop in between the two
• deselect
• select all from any of the two groups
• both groups contain third edgeloop verts.