It is heuristic cycle detection. You can turn it off from toolbar or from menu “View->Detect cycles” or “View->Do cycle detection”.
Cycle is something like recursion, both direct ( f() -> f() -> f() where -> means call ) and indirect ( f()->g()->f()->g()->f())
Callgring format (used in kcachegrind) is not saving full call stack, it stores only pairs caller-callee and it may be hard to restore longer cycles from this information