In general, when an object is damaged or disappears under the watch of a shomer, if it occurred due to an oness (extenuating circumstances) all types of shomrim except for a sho’el (borrower) are exempt. The gemara (Bava Metzia 42a) deals with the case of a shomer who put a sum of money and in a wooden structure so that the money was well protected against thieves but not against fire. Because of the possibility of fire, the shomer was guilty of p’shi’ah, which could obligate him to pay. Instead of fire destroying the money, thieves stole it. The gemara brings an opinion (which we accept) that the shomer must pay because in a case techilato b’p’shi’ah v’sofo b’oness (=tbpvsb) (the makings of negligence that end up with damage of oness), one is obligated.
Acharonim present two explanations for this rule: 1) Since the shomer was to a significant degree negligent, he cannot claim exemption due to oness. 2) At the time that he was negligent, the shomer became provisionally obligated to pay unless he would succeed in returning the object intact. Upon failing to do so, even if it ended up being due to oness, he pays based on the earlier obligation.
To better understand the explanations, we will discuss two Talmudic cases in this context. The gemara (Bava Metzia 36a) deals with a shomer who allowed an animal to run free, but instead of being stolen, it died of natural causes. Although this seems like a classic case of tbpvsb, the gemara cites different opinions on the matter. Abayei said he is obligated because "the air of the meadow killed it." Rava says he is exempt because "the angel of death does not care if it is here or there." It follows that, according to Rava, there has to be some circumstantial connection between the negligence and the eventual death.
The gemara (ibid. 93b) discusses a shepherd who left his flock and came to town, during which time wild animals killed some sheep. The shepherd must pay because of tbpvsb. The Rif says that this gemara follows Rava’s approach. Abayei would not agree with this conclusion because the shepherd’s absence did not have an impact on the outcome, even coincidentally. The Ra’avad says that even Abayei would agree with the latter gemara because, had the shepherd been there, he might have been possessed with a spirit of bravery and saved the sheep (see Rosh, Bava Metzia 3:9).
The Gilyon Maharsha (CM 291:9) says that the above machloket depends on the two approaches to tbpvsb. If the idea is that we consider that there was p’shi’ah based on what he did in the beginning, this would apply only when there is some connection between the original p’shi’ah and the eventual damage. If the idea is that an obligation is created that remains until the object is returned safely, then even if the cause for it not returning is unrelated to the shomer’s negligence, he still must pay.