Cyclopropane instability to addition reactions
You're absolutely right: cyclopropane's maximum ring strain makes it highly prone to addition reactions. Keep the following definition in mind:


  An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one (the adduct).


In your case:


  


Thus, it breaks down the ring into a straight chain, which makes the low stability of the ring a clear contributing factor to the reaction. 

References: Cyclopropane, wikipedia||
Addition reactions, wikipedia