If you haven't PK'd anyone or used the abyss, the only other ways I can think of are using an item with a perk that skulls you (think it's called Committed), or looting a warbands camp recently. Mandrith can also skull you if you ask him to, though something tells me that isn't the case with you since you have to actively opt into that one.
Or it could be a glitch too. I've heard of a few glitches and tricks being used to skull people in the wilderness even though they didn't do anything to normally get skulled. Kinda hard to say without knowing what you were doing though :s
^+^ Antediluvian of the Draculesti Bloodline ^+^
^+^ If the Gods see fit to curse us with the Blood, then we shall raise ourselves above them ^+^
to expand (slightly) upon A Mad Hatter's post about the 'committed' thing, that's an invention perk that may have come with another perk you put onto your equipment. Other than that, you might check out this wiki page to find the reason for your skull?
https://runescape.wiki/w/Skull_(status)Ooh, a light at the end of the tunnel...
...Do you think I should get off the tracks?