In general, matches can only be deleted if they were entered by hand and the person doing the deleting is one of the players. The most common reason your match has been deleted is that you played a friendly, had a good result and entered it into SquashLevels without agreeing to do so with your opponent. You have to remember that a good result for you is likely to be a bad result for them and they will get an email saying they have a new match result so they'll see it straight away!
The next most common reason for deleting matches is that the system thought it was a duplicated or an updated match. A duplicate match is same players, same date, same result, an updated match is the same but maybe one change such as a different result. Either way, the earlier of the two matches is auto-deleted leaving the the more recently one as the 'official' result.
And finally, it's worth knowing that admins can also delete results but that's very rare.