Just guessing:

Let's call you X.

My guess is that you X have a facebbok friend Y, and that Y has a facebook friend Z. So facebook can make a link between X and Z.

Option 1) Someone tagged X and Z (and perhaps Y) in a recent photo, so facebook decided to add Z to the suggestions of X.

Option 2) Facebooks was already showing Z randomly in the suggestions of X because they are linked. But since X doesn't know Z then X doesn't notice Z and just ignore Z. Now that X knows Z, then X recognizes the photograph in the suggestions.