Is there a way to detect what program is stealing focus on my Mac?
You can find the app that steals focus by saving the following code in a find_focus_stealer.py file and running it via python find_focus_stealer.py in a terminal.

Start running the script – it will print out the name of the active app every 3 seconds. Keep working as usual, wait for the problem to occur, and after a few seconds see the output in the terminal. You’ll have your culprit.

In my case it was a Symantec Antivirus background app (SymUIAgent.app).

#!/usr/bin/python

from AppKit import NSWorkspace
import time
t = range(1,100)
for i in t:
    time.sleep(3)
    activeAppName = NSWorkspace.sharedWorkspace().activeApplication()['NSApplicationName']
    print activeAppName


Credits to iMichael_ in this Apple Discussions thread.