It is caused by dust, pollution, salty deposits. When the first rains come, conductive path is developed that results in corona and arcing. When the rains wash out the pollutants it disappears. In areas where salt is used in winter to clear snow in the roads, such arcing can be seen on insulators in substations on the side of road- effect of salt deposits.
This problem is very severe in certain regions near to sea eg. Mediterranean sea-countries like Greece. In my state in southern part of India, we never have this problem though on sea coast. But as we move north, in Konkan coast such pollutants create havoc.
In areas with high pollution level, with frequent vetting from mist, rain or fog, RTV (Room Temperature Vulcanising) silicone coating is applied over porcelain insulators to avoid flash overs from pollution. This came in to market around 1985.
Principle of RTV coating is that a thin layer of RTV material adhering to ceramic porcelain and glass insulators ,vastly increases their pollution withstand as the silicone imparts hydrophobicity to what would otherwise be a hydrophilic surface. Moreover since the silicone contains low molecular weight (LMW) chains that continually migrate to the surface, the ceramic insulator will remain hydrophobic, even when covered by a layer of pollution..
RTV coating can be applied at factory or at site and recoating will be required only once in its useful life of 30 years or more. (IEEE Std 1523-2002 Guide for application, maintenance and evaluation of RTV silicone rubber coatings for outdoor ceramic insulators, CIGRE Technical Brochures 442 &478) In such wet locations, composite polymeric insulators will lose their hydrophobicity quickly thorough repeated discharges and hence the recommendation of RTV coating over ceramic porcelain insulators. At certain locations composite insulators may not be feasible, like areas where bird- pecking is a problem. But composite silicon insulators are effective against pollution in dry regions with less humidity levels.
Close spacing between discs can also create flash overs as Edison mentioned. But chances are rare for such mistakes nowadays. When I was designing HV transformer condenser bushings in early 1970s, we thought increased creepage length esp protected creepage (protected from rain through deep corrugated profile under the shed- it was termed antifog shed ) will help in polluted areas. Hence we provided minimum flash over height and provided as many anti fog sheds as possible to get increased protected creepage.Later research by NGK Japan and Nagoya university professors showed that this is wrong as the wet pollutants in protected area created more corona and arcing. They suggested easy clean sheds ie long and short alternating sheds( to break water path) and this is now universally followed after these studies were incorporated in 1986 as IEC 60815-3 parts Insulators for polluted areas. Rules for insulator design are given in this standard. So earlier utility specifications called for more than 50 % protected creepage length for insulators of CT, bushings and LA. Today it is less than 50 % !!