Answer by Alex Sumner:
1. Charles Darwin. His “Origin of the Species” caused a great undermining in traditional religion – but also caused people who did not want to abandon spirituality altogether to start exploring alternate belief systems.
2. The 19th century saw great advances in Egyptology – e.g. the translation of the Rosetta Stone – which raised the subject in people’s consciousness.
3. The death of the Duke of Sussex in 1843 caused, in England at any rate, an upsurge of interest in obscure masonic and quasi-masonic rituals. The Duke of Sussex had been the Grandmaster of the United Grand Lodge of England, and during his tenureship had fiercely resisted anything which was not the three Craft degrees or the Royal Arch. However when he died, Masons felt more able to explore. Some of the rituals which began to be practised are now fairly mainstream today: others however had varying degrees of esotericity, which in turn inspired the creation of deliberately esoteric orders.
4. The rise of Spiritualism from 1848 onwards caused a backlash from scholars of ceremonial magic, who claimed that the practices employed by Mediums fell short of the standards required by old grimoires. They also pointed out that Mediums tended to be a bunch of frauds, whereas real magicians were motivated by the noble ideals of Hermeticism and the Qabalah. One may argue that these claims may not entirely be justified, but it started a debate in occult circles which motivated a number of prominent writers to come out in favour of serious Occultism – such as Eliphas Levi, Paschal Beverly Randolph, Mme Blavatsky, etc. These in turn directly inspired the founders of the occult revival in the late 1880s.