Speculators and subprime loans backed by Wall Street led to a 50% spike in metro Phoenix home prices during 2005 and 2006, and then a record number of foreclosures from 2008 to 2012. 

The likelihood of all of these occurring is low in a typical subject-to transaction. its why way back in our day we did these with wraps CA has a debt instrument called an All inclusive Deed of Trust it wraps the orginal mortgage it is a second in function my dad had a company called CA wrap back in the day and did hundreds of them.. its de ja vu all over again.. he did this during the Carter interest rate spikes just like sub too is now coming to light in a big way because of interest rate spikes.. 

Online management platforms have come a long way but @Guy Yoes does a good job and does mention their Achilles heel - they deal with the 'back-end' of management.

The closure of a large number of public schools under Rahm Emmanuel and then Covid-19 are primarily what has contributed to recent spikes in the crime rate.

I just changed 1 sentence on my site, and I got a spike in conversions and CTR!!

But a spike in unemployment will definitely hit you hard.  

MFC is head of heels safer than it was even 3+ years ago.  

A big spike would indicate potential visitors from mailers.

Interest rates spike making a good deal not so good anymore.  

. - Equal Splitting (ratio utility billing) - What I've heard from lawyer friends who practice real estate law is a trending spike in resident issues.