probably climate denial
Garden centres often put bedding plants out too early & as they are grown in warm greenhouse conditions, it can be quite a shock to tender plants when they have to withstand the still cool temp's at nite. Imagine if you came back from a sunny climate & sat outside at nite in your bikini..you'd turn blue too! Having said that, beginning of May is usually a safe time to start patio balconies if you have south facing sheltered conditions.