Rashi has one pshat in this gemorah, but Tosfos explains the Gemara differently:
Furthermore, as with all situations of ohel, if the brim is less than a tefach wide, no concern of ohel exists. Howver, the style today is to wear wide brim hats!
Therefore, when the wide brim is up, there is usually no ohel of a tefach, however when the brim is down, there is a concern. Wearing the hat back may also avoid the concern of ohel. Thus brim up and hat back is how yeshivish bochurim wear their hats. THIS IS A LIMUD ZCHUS ON THEM (rather than the alternative that they are poshut azei metzach and ballei ga’aveh)!!
BTW- this might be why some chassidim wear spodeks rather than shtreimels. The spodek has no protrusion and therefore no ohel, while the shteimel protrudes and is like a brim of a hat.