The most common reasons I come across are: the owner died, can't find heirs.

And even IF there are violations leading to enforcement, the most common type of enforcement action in these violations is that the accused doesn’t admit to any wrongdoing but agrees to never do it again, LOL.  

Software is a common question on BiggerPockets.

Is this a common trend? 

Looking at pure spreadsheet math and common convention; if you pay down your mortgage you are effectively making that interest rate on your money. 

The market has definitely gotten more competitive, and it’s common to see BRRRR deals over $250K, especially given the high demand/appreciation in the area.

Common fees will include a set-up fee, a leasing fee for each turnover or a lease renewal fee, marking up maintenance, retaining late fees, and more.

Just simple common sense. 

It was common back in the day.

2-4 units are not that common, but builders are building them more now.