Why parallel LC oscillator producing very high voltage?
Imagine, if you can, that prior to opening the switch, there is (say) 10 amps flowing from V1 (the 12 volt supply) into inductor L1 (2.53 mH). The energy stored (W) in an inductor is: -

$$W = \dfrac{1}{2}\cdot L\cdot I^2$$

This works out at about 0.127 joules. When the switch opens, that energy is released by the inductor and will "fill-up" the capacitor C1 (10 pF). The energy equation for a capacitor is: -

$$W = \dfrac{1}{2}\cdot C\cdot V^2$$

And, if you calculate the peak voltage across the capacitor, that comes to 159 kV. That voltage will rise and fall sinusoidally as energy swishes back and forth between capacitor and inductor.

Maybe if you limited the current to something like 10 mA you would get a more realizable circuit.