The Output track (highlighted in red) has 15 volume filters on it all set to +3.5 dB gain. The Output filters are evaluated after the clip filters and track filters. Output filters are like a final modification to the entire project before handing data over to the encoder. This means all clips and tracks are being raised by 52.5 dB before preview or export, which is ear-shattering. R.I.P., headphone users.
The only assumption I can make, because I had that very +3.5 gain filter in my clipboard, is that for some reason I managed to apply that, multiple times, to the output somehow.