ok, but that happens only if you fill a soulgem... just to be sure: did you skill "Soul Lock" passive in Soul Magic? At Level 2 you have 10% Chance to automatically fill a Soul gem with "weapon skills" (light and heavy attacks or some attacks of the weapon tree)
edit: and sorry, i just copy paste the text, Change it to:
Code:
CENTER_SCREEN_ANNOUNCE:AddMessage(CSA_EVENT_LARGE_TEXT, SOUNDS.AVA_GATE_OPENED, "|c00FFFF Filled a Soul Gem|r")