Funny, I can feel the feelings of this urge, as I read. (Disclaimer: I am not actually a toddler.)

I can't dissect my feelings fully, but I think a large part is feeling the anticipation of shame/failure from the whole thing going wrong (e.g. the whole cup of dog food falling out) and as long as you haven't completed the task, that threat stays there. Spilling a few pieces draws attention to that threat - presumably to warn yourself off it. Throwing the whole thing down gives a wonderful relief: there is no more threat, no more striving for what might not be; what's done is done. But after that, with the stress gone and the relief come, you feel the clearness to start again.