Ok, so I agree with chlorine "normally" having the -1 oxidation state, by which I mean it is the most stable one. However, with +1 being unusually stable I won't agree with - the highest oxidation state of chlorine is +7 (perchloric acid and its salts). Now, there's differences in the relative energies and hence stabilities of oxidation states and hence it's possible for an oxidation state to transform to two others if this is energetically favourably - are you familiar with disproportionation? What could Cl(I) disproportionate into? This however only happens when you heat bleach.
At room temperature, when standing in aqueous solution a different reaction can happen - even though I didn't agree with +1 being "unusually" high, we can say it's high enough for it to be wanting to go into -1 - ClO- can also transform into Cl- and O2 (can you write the reaction equation for this reaction?), avoiding creation of a higher oxidation state as in the disproportionation.
Now back to what you said - all this means Cl(I) is a strong oxidizer - it want to get more electrons so it can go down to -1 (it needs 2 electrons / atom to do it). Spontaneity comes from the Gibbs free energy difference between the form it is in and the forms it could be in after the decomposition - are you familiar with Gibbs free energy? Also a catalyst is not necessarily needed for such reactions, if the reaction is fast in itself it does not need help (compare to decomposition of hydrogen peroxides, which needs catalysts for decomposition, as its decomposition reactions are quite slow). Does this make sense?