[Normally if an animal walks and happens to knock pebbles that cause damage, the owner is obligated is half damage. But what if instead of walking the animal kicked the pebbles on purpose? Is that a further unusual circumstance and so the owner would be obligated only 1/4]
The Gemara then asks why not answer it from the question of Rava who asked if העדאה "warning"applies to צרורות (pebbles).
To me the questions seem independent. But clearly the Gemara is thinking that you could not ask about העדאה (warning) unless שינוי (change) applied. But even that to me seems hard to understand because let's say Rava had answered "No. There is no העדאה by צרורות". Then we would also not know anything about if שינוי applies.
[One reason that is all seems hard to understand is that even העדאה did apply to pebbles, that would be because it would be considered in itself a שינוי. But that would not tell us anything if another kind of שינוי would be applicable.]
Rashi understands that Rava was asking specifically on the issue of a 1/4--that is. if העדאה applies. That would help to some degree but it still is curious that Rava does not mention anything about kicking (בעיטה). He only asks if העדאה is applicable to pebbles[[not to any specific case of pebbles. That would suggest his question was if an animal kicked pebbles three times and the owner was warned each time, then after the last time, he might be obligated full damages.