Question

Tosafot in Bava Kamma (2b, s.v. "ve-lo") raises the possibility that “another force is involved” is a stringency in the case of fire. The reasoning is that if so, a person has to be more careful, because he cannot identify and understand the nature of the fire, and he cannot defend himself by claiming that he was sure the fire would not spread. Therefore, if only fire had been written, I would not derive ox from it, because in that case the defendant can defend himself and claim that he knows his animal and was sure it would not go and cause damage.
 
And afterward Tosafot proves from the Talmud that “another force is involved” is not a stringency but a leniency. The reasoning is that one might initially have thought to exempt fire, since another force — the wind — is involved, and so he is not really at fault; he was not expected to anticipate such a wind. According to this, if only fire had been written, I would derive ox from it.
 
 
I wanted to ask: here both reasonings really seem correct. “Another force is involved” can be understood as a stringency, and it can also be understood as a leniency.
So in the end, which understanding is correct? Why did Tosafot choose the reasoning that it is a leniency and not the reasoning that it is a stringency? How do you choose between two opposite reasonings that are both valid?