This is something I heard today. I was slaying a pack of wolves and when I hit the last one with power attack my character yelled something to it (or it yelled something to my character but it doesn't seem possible) I was so surprised that I missed the words he used. The yell sounded same kind of you can hear NPC's yelling sometimes during the battles. So I'm wondering if it's possible that my character yelled or was it a bug which made a wolf to yell like npc?

More gaming has answered on my question, yes your character yells sometimes. Most common phrase my character yells to it's enemies is "On your knees worm!"

Really? All mine ever does is grunt.

THIS!

EDIY: I'm guessing a NPC was near by and you didn't notice.

My character used to yell, but then...

Are you sure you haven't used the console to add yourself to a faction you couldn't normally join? For example, I know I made a character that was part of the Forsworn faction(s) and every once and a while she'll yell "For the Reach!" or something like that. Other than that, in 200+ hours of playtime, I've never heard anything of the sort.