Automatically shift into 4 wheel low

I have a 2002 F150 that just recently had shifted into 4 wheel low while driving. After the engine was turned off, it was ok. I have not driven it since then. Any ideas? Could it just be a faulty switch? Any help would be appreciated.

When you say shifted into four-wheel low while driving do you mean actually went from high range to low range in the transfer case? If so there should have been clanging, clanking, engine revving up etc when it happened. If the truck did not actually go into 4 wheel low did the indicator in the dash show 4 wheel low?

I have never heard of this but then I am an old guy whose four-wheel-drives all have had manual transfer cases with two levers on the floor.