Most database actions are being logged, hence stored in the LOG file. The LOG file will grow when actions are being executed on the database and the current free size inside the file is not enough. In full recovery mode the space in the LOG file can only be re-used when 1. the action is completed and 2. a LOG backup is taken.

It looks like in your environment a large action (like ETL load or large update) or many small actions are being executed causing the LOG file to grow.

Check if you take regular LOG backups throughout the whole day and not only during working hours!