How does an RF transistor bias work?
The drain voltage will have a DC (average) value of 12 V because the inductor (at DC) is a dead short. There can be no other result here. Therefore it follows that the AC content of drain voltage must rise and fall about the 12 V average level. Maximum swing is nearly 24 Vp-p. This happens because of the formula for an inductor; as the transistor starts to switch off (on part of the AC cycle) the inductor tries to maintain current and will cause the drain to rise above the 12 V level.

This IS NOT restricted to RF transistors - all transistors will behave like this with an inductor in the collector/drain.

The worst case scenario is when the transistor is turned on then rapidly turns off - the resulting back-emf can destroy the transistor BUT, this is a class A amplifier and class A amplifiers behave with a little more dignity (despite the inductor being present).