I'd inferred that it was the company keeping them there, after hearing stories like this:

https://www.cbc.ca/radio/asithappens/as-it-happens-monday-ed...

This article goes into the actual reasons, the visas only came into play during the second month:

https://www.cnn.com/2024/05/18/us/baltimore-bridge-dali-ship...

Basically the company would get in trouble if the crew left, and they are too cheap to hire replacements, so they threaten them.