We don't actually know that this happened. The origins of this story come from Shihab al-Umari who is writing in the 1340s, about Mansa Musa's hajj to Cairo and Mecca 20 years earlier. In that account, al-Umari attributes to Musa the story that his predecessor and brother Abu Bakr II abdicated and set sail on an expedition.

There are numerous grounds on which to question this story. How much of this story is al-Umari's invention? How much is the invention of his informants (al-Umari never met Musa directly). If Umari and his informants are 100% truthful and accurate in their memory, was Musa accurate and truthful in his telling?

All of this is to say, we can not be sure of the historicity of Abu Bakr II's fleet.