If you travel to Morthal during this quest causing Brutius to spawn, fail to talk to him, then visit your player home, when you return to town Brutius may have wandered away over the mountains past Labyrinthian toward Whiterun. A trigger box is not configured to be player-only activated, passing NPCs and animals may accidentally advance quest stages, so Brutius's travel packages stop working.
The Brutius object itself has no packages. When its quest package conditions aren't working, it defaults to something higher up the chain, which is probably why he wanders away.
The first ([xx000B6B] ccBGSSSE001_MiscMorthal_BrutiusWait) uses two '=' conditions. Brutius shouldn't exist until the quest [xx0009E4] starts, so checking the quest is not running is silly, but not likely the problem.
The second ([xx000B67] ccBGSSSE001_MiscMorthal_BrutiusWaitAtFishingSpot) also uses two '=' conditions, stages 10 or 15. Instead, it should probably have only one condition < 30, so that it will run until he is dead. Better yet, it could just test for "not dead", which is probably faster.
The trigger box ([xx000B41] (places defaultSetStageTRIG [ACTI:00033F50] in GRUP Cell Persistent Children of [CELL:00000D74] (in Tamriel "Skyrim" [WRLD:0000003C]) at -8,16) see Alias_ContestLocationTrigger) is initially disabled. It shouldn't be enabled until stage 10 or 15. It should wait for the player to arrive (defaultSetStageTRIGPlayerOnly), instead of being triggered by whomever (or whatever) gets there first.
Finally, both ambush wizards set stage 20 on their deaths. So this might be another way the Brutius wait packages are failing? These wizards shouldn't be enabled until stage 15. (Needs checking.)
I don't know why they used stage 20 for the hidden wizards' deaths and stage 30 for Brutius' death. In my testing, I always kill Brutius first (actually Lydia always one-shots him).