Brennan was successful in his application for a head coach gig at the Titans having won the NSW Cup and the "superbowl". Meanwhile Griffin was extended another two years on top of the year he had left already. I don't know if Brennan was still under contract but how could we possibly keep him as a reserve grade coach under those circumstances?
Griffin's extension was rumoured well before the finals - once it looked likely we would make the finals. Given the slow start to 2017, and that Griffin was already contracted for 2018, extending him before seeing how 2018 starts seems a less prudent/commercial approach than usual - unless there was a contractual trigger.
My feeling is that Brennan was highly rated at the club. From Brennan's viewpoint, once Griffin is extended Brennan absolutely should move on. Also, very difficult for the club to sell "opportunity" to Brennan when the incumbent has just been extended to 2020.
However, if there was no contractual trigger and the usual prudent/commercial approach is followed, then Griffin's extension is pending the club's start to 2018. This may have been enough "opportunity" to convince Brennan to see how 2018 starts - and provide some insurance for the club.
Anyway, more my musings than anything else given what's done is done.