It was the '70s and the Bicentennial of the American Revolution was coming up fast. With massive social unrest, the OPEC embargo, and the widening division between the established ruling class and the people, we were a nation in turmoil.
The Democratic Party controlled Congress but leadership had lost control of the election process and quite a few districts and states were sending unapproved representatives to DC and they, much like our nascent revolt today, were on a mission to deliver their agenda.
We got the Civil Rights Act, Roe, HUD, HEW, Medicare, the EPA, etc. In response to these changes, we also got things like the Hyde Amendment, and the Congressional Budget Act of 1974. Sold as a means of addressing a problem that didn't exist, excessive Government domestic spending, this was the door through which the Senate got the power to override the House, Executive Branch, and will of The People, with no accountability.
The people had spoken and the establishment didn't like it at all. The very next cycle was the first of these shutdowns when President Ford didn't like how much HEW was spending.
Looking at the shutdowns that have followed reveals a distinct pattern;
HEW budget
the next three are all over Abortion
Carter (temporarily) took away the boondoggle that would eventually become the USS Theodore Roosevelt
Pay raise for Congress and abortion
Gutting domestic spending across the board
Building new Nuclear missiles
More for war, less for education
Domestic spending (The Flint MI poisoned water disaster was set up here)
Jailing poor people and supporting terrorism in Nicaragua
Domestic welfare
Funding Nicaraguan terrorists (The fairness doctrine was also lost here)
Domestic spending
Medicare
"Balanced" budget
Note that in addition to passing the rules that made them possible in the first place, in the first seven of these hostage situations, the Democratic Party controlled both Houses and the Oval.
Republicans get bad ideas and Democrats make them into law.