Short answer. Normally a fortuitous event excuses you, but Article 2147 names four exceptions where you answer for the loss anyway: if you undertook risky operations the owner never engaged in, preferred your own interest, failed to return the property after the owner's demand, or assumed the management in bad faith.
What the law says
The officious manager shall be liable for any fortuitous event: (1) If he undertakes risky operations which the owner was not accustomed to embark upon; (2) If he has preferred his own interest to that of the owner; (3) If he fails to return the property or business after demand by the owner; (4) If he assumed the management in bad faith.
Civil Code, Article 2147 — Gestor's Liability for Fortuitous Events. Read the full provision →
The starting point: accidents excuse
The general rule of Philippine law is that no one answers for a loss caused by a fortuitous event — an accident that human care could neither foresee nor prevent. An officious manager who takes charge of another's property is not made an insurer of it; if a genuine act of God destroys the property while he manages it prudently, the loss ordinarily falls where it lands. Article 2147 does not disturb that baseline. What it does is identify four situations in which the manager has forfeited the protection the baseline would otherwise give him.
The four situations that reverse it
The article states that the officious manager shall be liable for any fortuitous event: (1) If he undertakes risky operations which the owner was not accustomed to embark upon; (2) If he has preferred his own interest to that of the owner; (3) If he fails to return the property or business after demand by the owner; (4) If he assumed the management in bad faith. In any of these, the accidental nature of the loss no longer shields him — he pays for it as though he had caused it, because his own conduct is why the property was still exposed to the risk when the event struck.
The common thread
The four grounds are not a random list; each describes a manager who overstepped the honest, careful intervention the law protects. Chasing ventures the owner would never have risked, putting his own advantage first, clinging to the property after being told to give it back, or intruding in bad faith to begin with — all take the manager outside the role of a prudent, well-meaning steward. Having placed the property in a danger a faithful manager would not have, he cannot then point to the accident, because his fault, not the fortuity, is treated as the real cause of the loss.
What decides it in practice
Whether you are caught by Article 2147 turns on facts about your own conduct, so those are what a dispute will examine: what the owner customarily did with the property and whether you departed from it, whether your decisions served him or you, whether and when he demanded the property back, and the honesty of your reasons for taking over. If none of the four fits, the fortuitous event excuses you as it would anyone; if one does, the accidental loss becomes yours to bear. The safer course while managing another's property is to stay within the owner's own practices and to return it the moment he asks.