IF there was something clandestine on board, it could have been picked up at one of the previous ports of call, or during a ship to ship transfer. Reports suggest the Bella had transferred cargo ship to ship in the open sea.
That’s one possibility.
Other reports say that the “value” of a ship that is “seized, taken, impounded” exceeds all the profits from multiple “shadow deliveries”. This, all by itself, causes real problems for insurance, permits, etc.
Legal problems for the captain n crew add complexity and resistance to accepting “shadow” cargo.
This hurts Russia economically.
IMO, the following is the KISS reason.
Biden implemented “sanctions”, then did not prosecute those sanctions, cause lots of countries needed time to find alternative sources of oil n gas, and build infrastructure.
Now, most countries have accomplished that.
The BRICS countries thumbed their noses and skirted the sanctions.
And supported Russia in continuing its assault on Ukraine.
Seizing the ships enforces the sanctions .
Sanctions were intended to choke Russia’s ability to execute its "special military operation ".
DJT has said “stop the war n sign a peace treaty”.
Enforcing the sanctions pushes that agenda
The Bella was reportedly in the Caribbean, flying no flag. It fled, still no flag, and only put up a Russian flag later.
It had therefore provided a “reason” for officials to be suspicious, AND, didn’t have a country patron cover.
Apparently that made it fair game under international maritime law, to be stopped and boarded.
That IMO, again, is the KISS explanation.
The BRICS countries are complaining… N I don’t care.

ralph