As seen in the forum topic notepad "NOTIFICATIONS seem to cause deadlocks" here:
I've attached a test case. Switch to another tab and back and it should lock up. Note that I am using a combination of BUFFERACTIVATED and UPDATEUI; the problem I was originally having was threading launching some processing which did some styling and then a save/bufferactivated causing the deadlock.
For my script this means I cannot use notepad++ notifications at all, as it causes deadlocks just receiving the events if certain python code is running.