The spike in airline flight hijackings in the 60s and 70s was due to a spike in domestic political terrorism in general as well as a spike in awareness of plane hijacking as a crime and how to commit it. Many left-wing plane hijackers in the late 60s had the intention of redirecting the plane to drop them off in Cuba, where they generally were under the impression they’d be welcomed by Castro and the gang as friends and fellow leftists.
They were not; American hijackers were viewed with extreme suspicion in Cuba. Later hijackers followed some of the same general procedures as their predecessors headed for Cuba, hijacking the plane in order to escape law enforcement pursuit or to negotiate for political demand, but some of them also used the hijacking itself as an occasion for robbery — think Dan/”D.B.” Cooper, who in 1971 hijacked a plane via bomb threat in order to extort a ransom and facilitate his getaway with the money afterward by parachuting from the plane.
For what it’s worth, we’re pretty sure Cooper died either in the process of or shortly after his so-called getaway, but it’s one of several hijackings that captured the American imagination. What these hijackings generally were not were terrorist events in the sense that the plane was actually destroyed and its passengers killed, but airlines still took a tremendous loss in the process, passengers were understandably put off by the prospect of being held hostage, and hijackers sometimes didn’t seem to grasp that even international flights wouldn’t have the fuel to take them where they wanted to go in a single shot.
The FAA took measures to mitigate the sheer number of hijackings that took place, such as stepping up baggage screening processes, but the golden age of 60s-70s plane hijackings ended in the US with the implementation of physical searches of passengers in 1973.
Especially now post-9/11, it’s difficult to imagine how wide-open and flagrantly vulnerable American airports in the 1950s and 1960s were in terms of security; the law enforcement response to these hijackings was a large part of how that ended.
Part of the almost unfathomable frequency of American skyjackings in this era was due to their initial perception as relatively low-risk for the hijacker — airline policies advised flight attendants and crew to comply with demands and not put up resistance, and fellow passengers rarely put up a physical fight because the intention of the hijacking itself was not generally understood to be murderous.
Brendan I. Koerner’s The Skies Belong To Us gives an excellent account of both 1960s-1970s American airline hijacking in general and one specific politically-motivated hijacking of the 1970s.