SERP drop after 1 PBN link

Hi folks,

I have a site that's was ranking #3 for its intended main keyword. It was pretty stable for the last 3 weeks, with just 1 PBN link and 1 random blog comment link. I added a new PBN link 2 days ago, and within 24 hours the ranking has dropped to 6 or 7. I created the PBN site, which looks legit etc. and put the link straight away. Got the PBN site indexed into big G and then bam, hit.

Is this a usual procedure of the big G, or something more sinister? The PBN domain was never used…

SERP drop after 1 PBN link