Comment by WalkingDead

I guess that might have to do with load these search engines generate. Previosly in a thread we checked that comments posted in HN appear in google search after a minute of posting. That should create a good amount of load on HN. Multiply it for all the search engines in the wild, and PG probably have decided blocking those woun't do any harm. My guesss. Could be wrong, or it might be temporary or a mistake.