Short answer. Yes. Ordinarily a fortuitous event would excuse you, but Article 2147 makes an officious manager liable for the loss where he undertook risky operations which the owner was not accustomed to embark upon. Choosing a venture the owner would never have risked forfeits the excuse the accident would otherwise give you.
What the law says
If he undertakes risky operations which the owner was not accustomed to embark upon
Civil Code, Article 2147 — Gestor's Liability for Fortuitous Events. Read the full provision →
Why the accident stops excusing you
An accident that no care could have prevented normally frees anyone from liability for the resulting loss. But an officious manager loses that protection in the situations Article 2147 lists, the first being where he undertakes risky operations which the owner was not accustomed to embark upon. The point is causal: by pushing the property into a venture it was never meant to face, you put it in the path of the very risk that then destroyed it. The law treats your choice, not the fortuity, as the reason for the loss, so the accident no longer answers for it.
The measure is the owner's own practice
The test is not whether the operation was risky in the abstract, but whether it was a risk the owner himself was accustomed to take. A cautious owner who kept his capital in safe, familiar dealings sets a low bar; a manager who gambled that capital on a speculative venture has plainly gone beyond it. What the owner ordinarily did with the property is the yardstick, because the manager stepped into the owner's shoes and was entitled to run the affair as the owner ran it — not to substitute his own bolder appetite for risk and expose the owner to consequences the owner never chose.
The rationale behind the ground
This ground guards against the manager who, unbidden, plays with someone else's property as he might not dare play with his own. Having assumed the management without being asked, he owes the owner faithful stewardship, and steering the property into unaccustomed hazards is the opposite of that. The rule denies him the accident defence precisely to remove the temptation: if he could keep any gains from a bold venture yet blame an act of God for the losses, the incentive would run entirely the wrong way. Making him bear the fortuitous loss aligns his caution with the owner's interest.
What proves the operation was unaccustomed
Because everything turns on the gap between what the owner did and what the manager attempted, that gap is what a dispute will look at: the owner's established way of using the property, the nature of the operation the manager launched, and how far it departed from the owner's habits. Records of how the business or property was run before you intervened do the real work here. If the venture fell within what the owner ordinarily undertook, this ground does not bite and the accident may still excuse you; if it plainly exceeded the owner's practice, the fortuitous loss becomes yours.