To fix this switch class name with observable in css binding:
data-bind="css: {open : expandMenu}"
Also, you can use css binding is such formats:
data-bind="css: getClassForSomething()" // getClassForSomething must return css class (string)
and
data-bind="css: { open: isSomethingDone() }" // considering that isSomethingDone is viewmodel's method