"Browning out" or disengagement from work is usually a subtle phenomenon. So much so that the partners are unaware that they are losing the edge which got them where they are. But in this unforgiving legal environment it does not go unnoticed. Soon enough they are forced out.
There are myriad causes for the brownout.
At the top of the list is arrogance. Things had been going their way for a long time. So they settled into their MO. Therefore, they missed the signals that many of the variables were shifting. But their competition and adversaries don't.
Another is simply enjoying the good life too much. One former partner of a hybrid law/lobbying firm became consumed by brands. If he had been flexible when he was ousted he could have become a buyer for some upscale retailer.
Another cause is becoming overwhelmed by change and uncertainty. It's easy just to exit intellectually and emotionally.
The tragedy of a brownout is that in a downsized legal business it's difficult to have a comeback. Practicing law requires an infrastructure. Outside the system of law firms most former partners can't afford to configure that. That ranges from the right office space to the right networks of contacts.
How to avoid brownouts? Approach work as a newbie who needs to make a reputation. Forget influence, power and wealth. Those could vanish without the edge.