I have experienced some strange raft physics as well. I have a couple different questions that may lead to a possible reason.
Did you have any loose objects on the raft that could have caused a shift in weight? I assume this is probably not the case, given your description of events, but just wanted to ask to be sure. You are probably already aware that any movement of loose items placed on the raft may cause rapid and dramatic displacement of the usual direction.
Also, perhaps more importantly, do you use a boat motor or a standard sail to propel your raft? I have found, as you may have noticed as well, that walking against a sail that is attached to your raft can cause it to dip sharply in the direction you’re walking into it. While this can be helpful for fine-tuning a parking job at the docks, it can also be very disorienting if the effect occurs during travel and can certainly cause the raft to plunge uncontrollably until you move away from the sail.
Maybe neither of these is the case, it could be totally connected to the auto-walk function. In which case, maybe another player could provide more insight as I have never used that function myself.
Rafts can definitely react unexpectedly to storms in general, though you have likely already encountered that in your travels.
I edited to include the below link to a topic I created on the physics related to walking into sails, just in case.