This question already has an answer here:
At the end of Back to the Future II, a lightning bolt hits the DeLorean which sends it back to 1885. At the time though, the DeLorean wasn't moving; it was relatively stationary hovering in the storm. Yet, it's emphasised repeatedly that the DeLorean must reach 88 miles per hour (which becomes a key plot point in Back to the Future III).
So, considering the DeLorean wasn't traveling at 88mph when the lightning struck the DeLorean sending it to 1885, what is the in-universe explanation of how it travelled there?
Some other factors to bear in mind:
- If a lightning bolt was sufficient to send the DeLorean time traveling, then the 88mph business would be irrelevant
- Note that there are the firey tracks left after the DeLorean time travels in the '66' shape. The thing to note about this though is that we never see these before the DeLorean disappears. These tracks are always appearing after the DeLorean has traveled away. This would seem to indicate that the DeLorean did move, but following the activation of the time circuits.