A 50% increase in unique IPs isn't necessarily something which could cause downtime, but it certainly could be related -- which is the word Paul used.
For example: HN relies heavily on caching page data. If 70,000 robots suddenly started loading random items, it could easily cause both the jump in unique IPs and the downtime.
I wouldn't exactly call that immense but more importantly it an increase like that certainly doesn't seem to be a reason for a site like this to go down (I didn't see it was down I'm going by the headline).