Why was I upgraded?

Long story short, booked a flight in Premium Eco through a different airline but it's code share with LH. The class I booked is one that's exclusively no upgrade possibility. No point upgrades, nothing.

So I roll into the airport and my boarding pass gets updated. Upgrades to Business Class. Cool. Whatever. (First time upper deck on the 747 so secretly I am pretty chuffed) Get to the gate and LH is calling for people that want a paid upgrade to business or premium eco.

So why was I, with my "no upgrade ever sir" ticket automatically upgraded to business while others are asked to pay?

Don't have status or much history either.