I worked doing motor claims for a big insurance company.
In most cases like this insurance companies have 'knock for knock' mutual agreements. In unclear cases like this, it basically saves time, futile efforts and money finding out who was to blame. It's an understanding that most insurance companies have.
In theory, the car behind is always to blame, as you should be aware of anything insane that the car in front does and allow adequate room to brake or avoid a collision.