New to lightning too, but I'm suspecting that since your args don't mention a specific logger, the pl trainer will instantiate the default one. Excerpt from the trainer docstring:
logger: Logger (or iterable collection of loggers) for experiment tracking. A ``True`` value uses the default ``TensorBoardLogger``. ``False`` will disable logging. If multiple loggers are provided and thesave_dirproperty of that logger is not set, local files (checkpoints, profiler traces, etc.) are saved in ``default_root_dir`` rather than in the ``log_dir`` of any of the individual loggers.