Ok, lets try this. You are going to rob a bank, you have a plan to escape if the cops come by using a small explosive charge to blow out a large wall where an escape car is waiting. This is your back up plan right? cops come, all goes sideways, you set the charge, but instead of blowing out the wall, the whole wall and room collapses into debris and blocks your escape. Now you panic and look for a second way out. This was not your plan. This is you reacting to a situation gone wrong. This is the cat. The cat had no plan to back up. IT was forced to, because the pigeon did something it didn't expect. Its funny, because the cat a) didnt have a plan incase things went wrong (no good back up plan) and b) the result of having no back up plan, is the cat is forced to back up. Making the comment a pun. because the meaning(having an alternative or back up) is what he didnt have, so was forced to use the seconday meaning (reverse of movement, to back up) was the result of having no "alternate" plan.