First and foremost, aluminum foil is just plain weird to cats. Especially if they are used to leaping up onto the countertop without ever having seen it there before. It would be like if one day on your normal commute to work, you found that the road was suddenly covered in decorative rugs. You might find it a little odd, and would likely be a little apprehensive about driving over them. Maybe you would even consider taking an alternate route to avoid them. This combined with the fact that there is nothing like foil in the wild makes aluminum foil very peculiar to cats. They have no frame of reference on what exactly it is or how to deal with it.
Aluminum foil also has some unique properties that make it particularly alarming to cats upon their first encounter. When moved, bent, or stepped on, the foil has a very distinct high-pitched crinkling sound. Crinkling aluminum foil actually emits a sound so high-pitched that it reaches into the ultrasonic range. Our human ears are not adapted to pick up on ultrasonic sounds, so for us, this sound may not be so bothersome. But cats are far more sensitive to high-pitched sounds. Many of their natural prey like mice and rodents communicate using ultrasonic sound, so it’s no wonder cat ears are fine-tuned to pick up on such high-pitched noises! While we may not be able to understand exactly how the crinkling foil sounds to a cat, it’s possible that it’s similar to nails on a chalkboard for humans.
The sudden, high-pitched crinkle in combination with the unusually smooth texture of foil when the cat is not expecting it can potentially cause a sensory overload. According to an article published by the International Association of Animal Behavior Consultants, “Cats have an innate fight or flight response to fearful situations and would prefer to flee and hide from a perceived threat.” It is certainly possible that a sensory overload could create a fearful situation for the cat, which triggers their instinctual flight response - when in doubt, flee from the potential threat!
Another theory is that the foil replicates some of the same traits as water - it’s shiny and reflective like water. If a cat had not been exposed to foil before, and suddenly there was a large “puddle” of it on the floor or countertop, it’s possible that it could be mistaken for water at first glance. In general, it’s safe to say that cats prefer to stay dry. So if the foil is initially perceived to be water, then most cats would likely be hesitant to walk right onto it. It is difficult to say for certain if cats really are tricked into believing the foil is a body of water, but if it is something that they have never encountered before then it is likely that it simply presents a risk they want to avoid.