Hypothesis: this is your brain doing normal garbage-collection on pieces of information that you haven't accessed frecently (frequently + recently) - you're at the tail end of the forgetting curve[1].
Does your experience confirm or deny this? That is, do you have much more trouble with languages you learned 5/10/25 years ago than ones you were using last month? Or is the process random, and you'll forget features of a language that you were just using?
Alternate hypothesis: you're getting old, and your memory is starting to go - nothing to do with programming languages in particular, the process is just more noticeable there because we have to remember so darn many little things.
Less depressing hypothesis: your brain has gotten out of the habit of remembering those things. I used to be extremely bad at remembering names, until I started paying attention to them because I learned that people really like it when you remember your name. Now I'm great with names without even trying, and I'm convinced that you can also train yourself to be better at remembering other things (i.e. programming stuff) than your natural tendencies.
[1] https://en.wikipedia.org/wiki/Forgetting_curve