It is generally assumed that "Ba'alav Imo" is a Gezeiras ha'Kasuv" without any simple logical explanation. However, the Sefer ha'Chinuch writes that since, at the beginning of the term of borrowing, the owner came along and joined the borrower, it was the owner's responsibility to watch his own article. The Torah made this Halachah universal, such that in every case the borrower remains without liability even after the owner is no longer there.

Following his reasoning, we could add that possibly all the liabilities must take effect at the time of, and indeed through the act of, the Kinyan at the beginning of the term of borrowing. If those liabilities do not take effect at that moment, then they do not start later either.

D. Zupnik