I also have 2 cats and I know that cat behaviour can be inconsistent and erratic. But as allanmabel suggests, it is possible that a neighbour is feeding them. Cats like a change of diet - they are curious creatures and get bored with familiarity. Try a change of cat food. If you feed them on wet food, try dry. If you feed them on dry food, try wet. Try different flavours and ingredients.
But in the end, cats are weird things, aren't they? You can't analyse their odd inconsistent behaviour.