
The prime minister is a political enemy of the empress and her family. She works together with the second princess to make her the next empress and gain power by being the future empress supporter.
That was also her reason to marry her only son to the second princess, because as mother-in-law of the future empress, she would have even more political power, making her nearly as powerful as the empress herself.
But the empress knew this and crosses the prime ministers plans, by marrying the son to the political powerless third princess.
But now the second princess lost the war and a lot of political support, so she may no longer be able to become empress.
So the prime minister wants to stir up trouble for the third princess, who started to get supporters through her businesses with the merchants.
She sends presents to the third princess, if people would see the carriages enter the third princess house, rumors would spread, that the third princess now works together with the prime minister, and she would lose every favor with the empress who is the prime ministers enemy.
This would servery weaken the third princess standing and her chances for the throne.
That's why they have to stop the carriages from entering at any cost, showing, that the third princess does not work together with the prime minister.
All that political warfare is also the reason for Fai Yues tragic past. The second princess wanted him and the third princess absolutely didn't want him. Firstly because she liked her second sister and secondly because she was in love with the musician.
Wait. Stupid question but what’s inside the carriage that can cause them trouble? Assassins??