The Gemara mentions a case in which wife A should have gotten her ketubah before wife B gets hers. If, however, wife B grabbed it first we don't take it away from her. Why would we allow wife B to go against the din and get to keep it anyway? When do we find a case in which the halacha in mamones (monetary matters) is a certain way but if someone chops (grabs) his part first he gets to keep it?
The Gemara abounds with cases of Safek Mamon in which Tefisah (seizing) helps. The most famous is arguably the case of "Takfo Kohen" (Bava Metzia 6b). However, in this case it is not Tefisah which helps, but rather this opinion holds that the right of first payment is only when the collection goes through Beis Din. Outside of Beis Din, though, each individual creditor has a right to collect on his own without regard to preceding debts.