What triggers the SSRS to generate a log file or modify a current one?
The log in question I believe is 

Report Server Service Trace Log and it turned ON by default

https://docs.microsoft.com/en-us/sql/reporting-services/report-server/report-server-http-log?view=sql-server-ver15

I figured this by checking where the log is stored in the question.




  Trace log behavior is managed in the configuration file
  ReportingServicesService.exe.config. The configuration file is found
  in the following folder path:
  
  \Program Files\Microsoft SQL Server\MSRS13.\Reporting
  Services\ReportServer\bin.


if you want to disable this logging you can do the below.


  DefaultTraceSwitch    Specifies the level of information that is reported
  to the ReportServerService trace log. Each level includes the
  information reported by all lower-numbered levels. Disabling tracing
  is not recommended.   Valid values are:
  
  0= Disables tracing. The ReportServerService log file is enabled by
  default. To turn it off, set trace level to 0.


You can tweak this value based on your requirements.

0= Disables tracing. The ReportServerService log file is enabled by default. 
   To turn it off, set trace level to 0.

1= Exceptions and restarts

2= Exceptions, restarts, warnings

3= Exceptions, restarts, warnings, status messages (default)

4= Verbose mode


More info here