|Actually, I'm pretty sure these are toads, not frogs|
Amazed that this has happened yet again, I did a little bit of Googling and found that toads do in fact return each year to the same place to mate and leave their spawn (lucky me! all over my doorstep!). Of course, last year's spawn was promptly scrubbed off the doorstep with bleach, so no little toads emerged. I don't know what these guys are thinking. Hoping to break the cycle and to avoid accidentally stepping on the little guys, we (and by "we" I of course mean Roger) very gently relocated the toads to the forest where their babies at least stand a chance of hatching.
In my Googling, I also discovered that toad and frog mating happens with a vengeance in certain locations in Luxembourg, and the city actually puts up signs blocking off roads where the little critters tend to congregate, not only for the safety of the animals but also for drivers. Squished toads, apparently, are very slippery!
Also, in some parts of France, they build "crapauducs" -- barriers that guide the animals into holes where they are trapped until they can be removed to safe places. ("Crapauduc" has to be the best name ever! Crapaud means toad in French).
More info here (but in French):
Now we're just waiting to see if they return or if others are going to turn up. I can almost guarantee that no toad has ever been hatched on my doorstep, so it is a complete mystery as to why they all congregate there...