Looks like no one seems to know the real reason for the detention. Vincent might not have not known then but he would have by now. Anyway here is the story behind all this.
The authorities have always monitored people who focus on worker's right since the days of SATU and Lim Cheng Siong. Shortly after Jurong Industrial estate was built, the Catholic Church and laypersons associated with it began to work with workers in an outreach programme. One of those was a foreigner and I think he was either a Hawaiian or Japanese. Vincent over time became his pupil. After watching them in the 70s, the foreigner was deported. In the meantime, a Catholic nun began creating waves in the old Nazareth Centre in Bukit Ho Swee with the poor and the less able. The authorities became weary again but they could not deport her aas she was a Singaporean and a Minister intervened threathening to arrest her. She was sent back to the convent.
In the early 1980s, Vincent's path crossed with a number of law students in NUS over old man's procreation policy. The students wanted to protest and the authorities stepped in and the compliant NUS Student leader by the name of Viv B and his number 2 called Simon Tay obliged.
One of them after graduating in law went on to become a student organiser in Singapore Poly Catholic student movement. The law connection also brought a number of practicising lawyers into the picture at the Geylang Centre and became involved with Filipino maids and their rights. The Liberation Theology and the shadows of Cardinal JaimiSin began to loom over old man.
The authorities kept watch and saw a link into worker's party. This was when old man lost his cool. It became clear that the Hammer was done behind the scenes by well educated people including a Harvard Grad. To make matters worse, one was married to the younger sister of Juliet Chin. And then the whole thing exploded when the link to Tan Wah Piow was made.
Old man naturally panicked and got them detained.
There was no plans for violence, no weapons acquired or stored. There was no plans to incite the masses. There was no shred of evidence to support a mass movement or even a movement to agitate.
Even Tharman then not associated with the PAP was questioned for a week.
In the course of the interrogration, something completely unrelated surfaced - Francis Seow who was acting for some of the detainees was approached by US diplomat to stand for elections against the PAP. He was then detained. It was revealed by one of the lawyers who was detained.
In fact, it was Seow's arrest and the deporting of Hank Hendrickson that was big fish that came out of the whole Marxist Conspiracy. A foreign power persuading lawyers to stand against the PAP, providing them funds and guarantee of an imunity. The discussions were held at the basement cafe along Robinson Road well known to lawyers.
To cut a long story short - the detention had no basis. There was no evidence of anything except protect worker's right which the arseholes on NTUC never did. Ask Mdm Halimah how much she earns a month. You guys will be shocked.