Why do bougainvilleas shed their flowers?

Are you sure they are shedding flowers ? Many people don't realise that the flowers on a bougainvillea are not at all conspicuous. They are very, very pale yellow in colour, Trumpet shaped, and very narrow in relation to their length. They are truly tiny by comparison with the brightly coloured objects that look like medium sized and brightly coloured flowers. These beautiful "flowers", often as large as an inch or more across are not in fact flowers. The "petals" are actually a modified form of leaf called a bract. Plants do shed their leaves, and this includes bracts. If you want to look again at your bougainvilles, get up close and take a magnifying glass.