...and what is the cause? Tank corrosion?
Yes, fuel tank corrosion.
Most regular readers of this site will probably be familiar with why this happens but for the uninitiated:
Any machine that has a (especially metal) fuel tank that is not expected to be used for some time should be left with a FULL fuel tank.
This is because any space in the fuel tank that is not occupied by fuel will be occupied by air; air that contains moisture.
With temperature changes that moisture will condense out onto the inside of the tank and then drip off and pool in the bottom of the tank which will then rust.
These pictures perfectly illustrate the result!