kicks into 4x4 unexpectedly

[oldiesel]

my 04 3500 auto.. kicked into 4x4 without me using button..cant tell you if it did it at speed or while I was coasting or stopped..there was a caution saying "sevc 4x4" and when I turned off the hi way I noticed the front end was locked up..to get it unlocked I tried switching in and out of 4x4 and that didnt work..got it out by shutting the ignition off and restarting back in 2 wheel drive mode..to get to the question now..how does the 4x4 actually work..is it an actuator in the transfer case and if so how does the front wheels lock up.. has anyone had this happen to them and what was the fix..about 300k km on unit..transmission replaced 20k ago..thank you.