You have exceeded your daily logs GB/Day licensing limit within the last 7 days

Dear,
Please check the below link: 
https://community.fortinet.com/t5/FortiAnalyzer/Technical-Note-Minimizing-logging-from-FortiGate-to/ta-p/198018

Fortianalyzer does not provide any info regarding this - not what logs are in excess, nor from which Fortigates (the limit is calculated as a cumulative log intake over some time, if serving multiple FGTs). We cannot even know for sure what happens to those excess logs - from Fortinet viewpoint, it is undetermined behavior, so they cannot say if those logs are being dropped or still logged.  

In other words - you don't really have means to answer "why this happens", only as @nnair mentioned, you can try and reduce volume of logs sent to the FAZ until this error message disappears.