Accelerated Oxidation Of Iron When Coating Breaks
The main factor for this phenomena is the the values of the standard electrode potentials for of the metal ions. 

The UC Davis ChemWiki page Corrosion, uses this scenario as an example:


  the presence of the second metal can actually increase the rate of corrosion. The values of the standard electrode potentials for $\ce{Sn^2+}$ (E° = −0.14 V) and $\ce{Fe^2+}$ (E° = −0.45 V) show that $\ce{Fe}$ is more easily oxidized than $\ce{Sn}$. As a result, the more corrosion-resistant metal (in this case, tin) accelerates the corrosion of iron by acting as the cathode and providing a large surface area for the reduction of oxygen


Tin is therefore more of a noble metal than iron - a galvanic corrosion chart is shown below, showing that tin is more 'noble' than iron (Image source):



The significance of tin being more noble than iron is that, according to the UC Davis ChemWiki page Electrochemical Corrosion, it can act as a depolariser, removing more electrons than the water/air alone - hence, accelerate the corrosion process.



Instead of the copper used in the example, it would be tin in your example.

A somewhat simplied illustration of the process is shown in the diagram below: