This was the same thing that happened that time, SmRT trains
often stop in the middle of Pasir Ris and Tampines stns while
commuters on board before moving again1. I used the
same reason to explain the above. nowsadays, instead I would
see a train stop suddnly while on a bus or car. The last time that
happened was when I was taking a bus and while traveling below TPE,
I saw the train sitting jjust above the train tracks.
1 The last time I was sitting on the train when this happened
was in jan.
[Note: This is not related to the threadstarter's question, but
clarifications for langer68's post.]
This is only caused when that train that stopped is waiting for
one of the trains to clear the station. It happens 70% of the time
during peak hours, and happens at Marina Bay and Joo Koon too.
Like for example, there's two trains in the station, the train
could not enter the station as both platforms are filled. So this
train (the train that's outside the station) waits for one train to
depart before proceeding.
Another case is when there's one train departing Pasir Ris at
platform A, this train (that's outside the station) would enter
platform B, however, as the train at platform A has already set the
route and is departing, the train outside would have to wait till
the train crossovers, and after the signals and tracks have been
set, then the train would proceed.
(Correct me if I'm wrong, cause I know my percentage is
wrong, 
.)