The more we discover about DNA, the more we understand that junk DNA (pseudogenes) are not so junky after all. I suppose that instinct is a result from the program of ncRNA (non coding). Wes Warren of St. Louis University Genome Sequencing Center has discovered ncRNA to be the controller of the brain for songs of finches.
Purely supposition on my part, but it is the desire to sing, which engages the ncRNA to put brain neurons into action, thereby engaging the body to function accordingly to express the original desire.
I believe it begins with desire. And the research of Warren suggests that desire is not a function of brain activity, rather it is something else altogether undetermined that moves the ncRNA into action first.
So… I can easily suppose that your cats desire to relieve pressure in the bowl is similarly engaged. Or that @Disc2021‘s desire to satisfy the pangs of hunger, sleep and coldness are also engaged in this manner.
The question is… What is desire?
I propose that desire is a non physical agent, never to be demonstrated by the empirical sciences. Promoting a non physical agent at play in all of this gives us cause to consider that living beings are more than mere flesh and blood, or the programs that run the hardware. There is a good argument to suggest a desire element controlling it all externally.