Did you go check the specific activities that those PR's came from?  you can click on each of the PRs and see the activity they are coming from.  It might be a single file that you have that has bad GPS data.  If your GPS signal was poor, it could result in those anomalies.  Sometimes you can just delete the activity if it isn't correct or you can crop out the bad part of it if it is right at the start or end.  I see a lot of these in other people's files where they recorded an indoor activity but left their GPS active.

I have a lot of these also, caused either by bad GPS, or old activities imported from other apps.

Tap the ⋁ button to expand the offending activity, and if you’re a subscriber, you can exclude it from Best Efforts calculations, or enter the correct time for that distance:

Strava should just make it easy, and apply their leaderboard algorithm to best efforts as well.

Hi, when doing a "cleanup" in my unrealistic "best efforts", the Correct Distance feature helped quite a lot - I didn't have to exclude most of these efforts, as the distance recalculation triggered by that feature fixed the pace as well. 

Note, it's only available on the Strava website (you need to open the particular activity there), not in the mobile app.