“A guy walks into a bar…”  is a typical form of what has been called the “bar joke.” A anagramist version is:
     
“An anagramist walks into a bar, and the bartender says, ‘Why the clean fog?’”
     
The joke is based on the popular bar joke involving a horse:
       
“A horse walks into a bar and the barman says, ‘Why the long face?’”
   
“Why the clean fog?” is an anagram of “Why the long face?” The joke was posted on Twitter by Loon Liquors on January 14, 2013.
     
         
Twitter
Loon Liquors
@LoonLiquors
An anagramist walks into a bar, and the bartender says, “Why the clean fog?” #barjoke 4.5 days left to pre-order - http://indiegogo.com/Loonshine
1:37 PM · Jan 14, 2013·Twitter for iPad
   
Twitter
Yash
@bonnybridgelost
An anagramist walks into a bar, and the bartender says, “Why the clean fog?”
7:03 PM · Sep 28, 2013·Twitter Web Client
   
Twitter
Rock Slyde
@chasaway
An anagramist walks into a bar. The bartender says, “Why the clean fog?”
7:29 PM · Oct 1, 2013·Twitter Web Client
 
Twitter
Jim Corbridge
@MrBonMot
An Anagramist Walks into a bar & the barman asks “Why The Clean Fog”
5:18 AM · Mar 3, 2014·Twitter Web Client
 
Twitter
heather b
@heatherbussing
Replying to @MiguelNogueras @lawprofblawg and 7 others
An anagramist walks into a bar, and the bartender says, “Why the clean fog?”
GIF
3:28 PM · Oct 20, 2019·Twitter for Android
 
Twitter
Jim Corbridge
@MrBonMot
An anagramist walks into a bar & the barman asks
“Why the clean fog?”
11:50 AM · Dec 6, 2019·Twitter Web App
 
Twitter
Gemma
@Gemmapolyglot
The world anagram champion walks into a bar.
The bartender says: “Why the clean fog?”
3:10 PM · Jun 20, 2020·Twitter for iPhone