It does sound somewhat counter-intuitive - one might think that the symptoms of asthma would be exacerbated in regions with lower levels of O2, as in altitude, or the less humid colder air might act to dry out the airways and hence cause greater irritation.
If however, your asthma is a consequence of an allergy, the reason your symptoms dissipated when skiing might have less to do with the altitude and more to do with the increased purity and lessened allergens in the atmosphere - and when you came back home, your asthmatic symptoms gradually increased as your system became once again exposed to the particular allergens that trigger your asthma.....