SquashLevels is a global system and part of its nightly processing is to calibrate the levels of players across clubs, regions, countries and even time. This process chips away at the levels of players as groups, whether up or down, with the goal to ensure that player levels are equivalent across the board. The changes each night are very small and, for those playing regularly, they will largely go unnoticed but, if you take a break and then come back, you can find your level has changed over that time and it may be more obvious.
New historic matches
Another reason for your level to change is if we have received a number of historic matches that may have had an effect on your history directly or maybe indirectly via some of your opponents. We continue to connect to new systems and sometimes can pull in very large numbers of new results which are likely to have some effect, dependent on how close you are to the players involved. When the volumes are very high we will pre-calibrate on a development system before bringing them across but we probably can't eliminate these knock-on effects. This may be what's behind an unexplained level adjustment.
Another possibility is that one or more of your opponents had a duplicate profile which has now been merged into a single profile and that has caused their level to change. This will have an indirect effect on your level.
In reality, everyone's level is relative to everyone else's level and the more historic results we read in, the more impact they will have.
One final note, that level changes are relative so a 1% change for a 10,000 level player will be more significant (100) than a 1,000 level player (10). If you are hovering around a major threshold (such as 10,000) it can seem quite a drop if you're now down into the 9,900s even though it's only 1%!