Having postgres consume 100% CPU probably means that a report is running. It might be other things but cannot say without properly analyzing the logs from the system.
Keep in mind though that this is not necessarily an issue depending on the specs of your machine.
In Linux, 100% CPU is not the same as Windows. What you see there means that the process is consuming 100% of 1 CPU core not the whole CPU.
So if you have 10 CPU cores assigned to this machine, this would translate to 10% of total CPU usage.
If you use HTOP instead of TOP, it will show you more information which will make this more clear.
Do you have scheduled reports on this system? And are they scheduled to run at about the time you saw this?