xzjv has been working on something for this.
Essentially what's going on is:
Bosses are highly weighted in the bot logic [It's a boss! Go there now!].
When they first start to spawn, you can't actually do anything to them but your bot wants to interact with them.
Since it can't, the bot defaults to moving to them to give a quick cuddle.
Once the spawn is complete, the guardian is already within your damage range and you start DPSing while standing on the boss.
If you look at the BETA changelog for Trinity .700 [See Discord #Builds channel], xzjv added something to attempt to keep you away from bosses while they are spawning. Take a look at getting a new Trinity BETA after that one and see if you notice a difference.
Any feedback on these changes would be appreciated.