Yet Another Brent Council Executive Change

I understand that yet another member of Brent's Council Cabinet has stood down.  This time it is Cllr Roxanne Mashari.  Prior to Cllr Keith Perrin in September 2014, it was highly unusual for councillors to step down from the Executive.  Now, Cllr Mike Pavey, Cllr Wilheimina Mitchell=Murray and Cllr Roxanne Mashari all appear to have stood down having only been elected in May this year.  What exactly is going on?