Spike in RTL Inverter
Because that's how capacitors work &mdash; they resist a change in voltage across there terminals, until a current flows to add or remove some charge.

In this case, you have a sudden positive step on the base of the transistor. The B-C capacitance couples this step to the collector terminal. In order to reach equilibrium again, current must flow out of the collector terminal, and the only place it can go before the C-E conduction starts is through the load resistor R2. This forces the voltage on its lower terminal to briefly rise ABOVE the supply voltage on the other terminal.