Il mistero del limone sul balcone (The mistery of the lemon on the balcony)

An African proverb says that if you are a date tree, you can't produce walnuts* - meaning you cannot change your nature. But those who said this apparently irrefutable said have probably never experienced what we experienced a couple of weeks ago...

On our terrace we have several plants, including a lemon tree - which, on our own word, produces juicy and tasty lemons. 

Well then, a couple of weeks ago we noticed that the fruits on the tree were too orange to be lemons. We got closer to watch better, and - strangely - they looked more like mandarins than like lemons. We tasted them, and - we could not believe it - they tasted exactly as mandarins. 

We were puzzled. Lemons that look and taste like mandarins.

Inter-pollination? Pollution? Climate change? GMO??? Can anyone help us in solving this mystery???...

(*) The fruits in the proverb may actually have been different, I can't remember - but the concept remains the same.