We don't know why they were laughing. I suspect there was some heckling, back and forth, which is common from ALL parties, and they were reacting to that.
Should they have been listening? Yes, but there are long stretches where they are engaged in their own side discussions and/or heckling, etc.
It happens in every legislature. It happens with all parties.
I would like to know the full context. What happened before this clip, etc. Unfortunately, Hansard does not record the background.