I've seen this happen a couple times now. only seems to occur with creature races and this is from a brand new fresh game so I have installed or updated any new mods.

One instance I had a dwarven centurion pursuing me but wouldn't attack... Also had a troll running around aggresive toward me but again would not attack.

I use duel combat 7, ERSO enhanced enemy AI, Revenge of enemies, SkyTest and a few encounter mods such as immersive creatures, Ultimate Deadly encounters & OBIS ... Plenty of other mods but nothing else that touches AI, combat or encounters

I use BOSS and bash patches.

Any clue what may be causing this to occur?

This can be a symptom of not waiting for cell reset when adding or updating a mod that edits creature lists. It seems that when the engine expects one spawn (as already allocated in your savegame) but gets another (as a result of the addition or edit to the creature list) oddities occur, such as creatures being passive when they should be aggressive, or sliding/skating unanimated creatures in a crucifix pose.

You could try waiting out cell reset (Immersive Creatures has MCM options to prepare for cell reset by changing respawn rates) and hope that it helps - on most occasions waiting for cell reset after the issue occurs will clear things up, but sometimes once the issue occurs it persists for that savegame.

Bashed and reproccer patches can also confuse matters - these should also be taken into consideration and rebuilt at the appropriate times when waiting for cell reset before/after adding or updating mods that edit creature lists.

However, cell reset isn't the only possible cause - it could be due to the engine being overstretched by a combination of your other mods and system specs, with spawns refusing to behave as expected.

I don't have any detailed technical explanation for exactly why or how this happens, but I've seen issues like this on and off ever since I first made my SkyMoMod replacers, and waiting for cell reset before and after updating does seem to prevent the issue from occurring in the first place for a wide range of mods that edit creature lists.

Is a cell reset required after starting a new game with all the mods installed? As this is the test I've currently conducted. I have all my mods installed, start a new character and then it goes through the enabling process of all the mods in game... after all the mods are enabled during the new character sequences and after helgen, is it still recommended to do a cell reset?