Two completely different perspectives on this trade; one stemming from a disgruntled fanbase, the other focusing on the possibilities that having ~$50m in cap space can offer...
These guys and their echo chamber seem to think Dirk opted out to "threaten" Cuban, hold his feet to the fire and force him to succeed at building a team around Dirk (as if Cuban's failure somehow stemmed from laziness? I dunno)... They even talk about the possibility of Dirk leaving and which teams he might go to:
What does Dirk Nowitzki's opt-out say about the Mavericks' past -- and future?
Whereas this guy seems to think Dirk opted out so that Dallas could use the extra cap space to absorb trades (Cousins?) or sign max players (Parsons/Howard?), then Dirk signs for whatever money is left in the end:
The Plan Behind Dirk Nowitzki Opting Out
I dunno about you guys, but I know which article sounds more like Dirk to me...