Trip hop originated in the '90s in Bristol, England, during a time when American hip hop was taking over Europe's music industry. British DJs decided to put a local spin on the international phenomenon and developed hip hop into a different style, marking the birth of trip hop. The originators in Bristol developed hip hop with a laid-back beat (down tempo). Bristol hip hop (trip hop's predecessor) is characterized by the emphasis on slow and heavy drum beats and a wide open sound that draws heavily on acid jazz, Jamaican dub music and electronica. Massive Attack's first album Blue Lines in 1991, is often seen as the first manifestation of the "Bristol hip hop movement" (known as the "First Coming of Bristol Sound"😉, but in fact Massive Attack drew heavily on the pre-existing British hip hop scene, and their sound is remarkably similar to that pioneered earlier by Marxman, an Irish-Jamaican hip hop crew that was popular in the UK in the 1980s.
Incidentally Massive Attack, Portishead and Tricky all had a common history. Massive Attack's three members used to work with Tricky, under the group "The Wild Bunch" (headed by Nellee Hooper in 1982), explaining why many Massive Attack songs feature Tricky. Portishead member Geoff Barrow also previously helped produce Massive Attack's "Blue Lines."