Personally, I'm prone to taking on technical debt when under time pressure or feature pressure. I think of it like my house: if I'm too busy, I get sloppy and leave messes. If I stay busy for too long, the messes stack up.

One complicating factor, though, is the age of the code base is a factor, too. Keeping up with new runtimes, new frameworks, and new ways of doing thing takes time. This is especially a problem with things built in the new hotness, because there the culture is one of rapid iteration. So if we really want side projects to last, I think we have to choose boring technology: http://mcfunley.com/choose-boring-technology