In the 40 years after the end of World War II, over four million immigrants, largely of European origin (thanks to the seriously racist White Australia Policy that was only dismantled across the 1960s and 1970s) came to Australia. This was a very large amount of people considering that Australia's population in 1945 was about 7 million. Perhaps unsurprisingly, given Australia was once a set of British colonies, the most common origin of those immigrants was the UK. In the 1950s, about a third of immigrants came from the UK (according to Jock Collins' book Migrant Hands In Distant Lands).

One of those 1950s British migrants was a young Ronald Belford 'Bon' Scott, who was born in a small Scottish town called Kirriemuir in 1946, and who emigrated to Australia at the age of 6, in 1952. According to Clinton Walker's biography of Scott, Highway To Hell, Bon was musical from a very early age, with a fondness from drumming on anything and everything. Isa Scott (his mother) is quoted in Highway To Hell as saying that when the pipe band would march through Kirriemuir on a Sunday night, '[Bon] had to get and march up and down with them'. So for an adult Bon Scott, the bagpipes were a memory of a very distant childhood.

AC/DC as a band had existed before Bon Scott joined the band; he was not their first lead singer (that was one Dave Evans, who is present on their earliest recordings). They were originally primarily a vehicle for Malcolm and Angus Young, the younger guitarist brothers of George Young. George had achieved some international success as a member of the Easybeats, an Australian band of the 1960s best known for 'Friday On My Mind'. The Easybeats had inspired in Australian fans what was known as 'Easyfever'; the Easybeats were bigger than the Beatles in Australia for a few months. After that band broke up, George Young and Harry Vanda (the co-writers of 'Friday On My Mind') became known as producers and songwriters, writing hits like 'Love Is In The Air' by John Paul Young, and 'Evie (Parts 1-3)' by Stevie Wright (the former singer of the Easybeats). Another band that Vanda and Young produced in the 1970s was AC/DC, featuring Young's younger brothers on guitar. In its early days, AC/DC had a more glam rock feel before Bon Scott and his particular masculine viewpoint and energy entered the band.

Like Bon Scott and his family, the Youngs were from Scotland (Glasgow rather than Kirriemuir), and they emigrated to Australia in 1963; the Easybeats as a band more or less started in Villawood's immigrant housing for new migrants.

Walker doesn't discuss the genesis of the bagpipes on that track in Highway To Hell but he does indicate that Bon Scott saw the presence of the bagpipes on the track as an indication that AC/DC was a band where he as a singer could be himself, rather than have to put on some kind of pop act (as he had to in previous bands he had been in - by 1975, Scott was close to 30 years old).

In an interview with the band's bass player at the time, Mark Evans (here), Evans suggests that the idea of putting bagpipes on the track was actually (Scottish-born) producer George Young's, and that Bon Scott then piped up saying he played in a pipe band in his youth (though he actually only played drums in the pipe band). The job was then given to him, and he figured out playing the bagpipes well enough to play the part in the studio with the help of some tape loops; he would subsequently play the bagpipes when they played the song live.

Because 'Long Way To The Top' became a fairly iconic song in Australia (e.g., the big ABC documentary on Australian rock & roll was titled after the song), bagpipes also became something of a tradition amongst Australian acts of using bagpipes on tracks; other Australian rock songs with bagpipes include John Farnham's 'You're The Voice', The Church's 'Under The Milky Way' and Jebediah's 'Fall Down'.

So essentially, in AC/DC recording the song in 1975, you have a situation where one of the producers (Harry Vanda was also an immigrant, but from the Netherlands), two of the guitarists and the lead singer were all born in Scotland (the rhythm section in AC/DC in 1975 were Australian-born). So perhaps it's not surprising that bagpipes were going to start blaring at some point.