What is the cause of this? 

That statement alone will cause you to move too quickly and take too much leverage which minimizes those solid investments you already have.Ask yourself, why do you want to scale faster?

I also learned of landlords (at least in one case) that are getting HARMED by high volumes of text spams and calls from investors.  

Unfortunately these do cost more and tend to dry out in the can, causing waste.

While rental demand is going down, there continue to be more people purchasing properties for personal use in turn causing there to be an overall shortage of rental units available in season.

Lastly with that said amount I’d say you put most into liquidity for your health reasons and start out with something you can be able to afford when your health needs best part of so you don’t start worrying much on what to do further causing you higher health risks because RE isn’t a get rich quick…..That been said, the ball still in your court….!!!

They aren't too bad if you automate things.As others have mentioned, reliable cleaners that do a good job are critical to your success.Changing regulations, saturation and a slow economy (like now) can cause you hardships.

Is it valid to evict her with just-cause? 

AB1482 says:“This property is not subject to the rent limits imposed by Section 1947.12 of the Civil Code and is not subject to the just cause requirements of Section 1946.2 of the Civil Code.

The cause of the leak is that when they built the shower, they tiled it incorrectly.