I believe it has something to do with 3rd parties. I have searched for a particular flight using skyscanner and then they have sent me to Wizzair site. I saw before the end of the booking this Surcharge.
I have opened a new browser in Icognito and I have searched for the same flight directly on Wizzair (no more 3rd parties this time). Afterwards, the surcharge did not apply again anymore.
Hope this helps, Regards