The insignificant nobody me doubt that it was due to racism. Those who called such incidents do not know what really is racism, as practiced in other nations, where it will not be the dark skinned Human that will get a seat, but will be verbally abused to get off the train, supported by others, even station managers, with protests being held to ban such fellow Humans from taking the train.
Here in Singapore where ALL - local or foreign guests - are equal, the only thing of color that interest us is the color of the notes in one's wallet, & not the skin. The reason why the lady got up and moved away could be many reasons - such as stereotyping, smells, incessant talking on phone or to others, loud music from one's phone, selfish manspreading, etc, & may even because she respects the elderly & got up to let another take the seat. Thus, there is no need to be too sensitive. This is Singapore.
To the one whom feels offended, just know that you are an equal AND do not have to worry about HOW others think about you. No one else can live your life, only you can. So long as you had been considerate, observed norms, the seat is yours, be comfortable & have a pleasant ride. We Singaporeans worked hard everyday, have MORE on our minds than be bothered about minor issues such as incidents of these manner.
To the one who got up, it may had been various reasons as listed. It is your right to do so, & need not worry about how others feel or think about you. It's your life to live. Just be comfortable & have a pleasant ride, than to be uncomfortable for the rest of the journey, which may take more than an hour in the enclosed space from Pasir Ris to Tuas. We have MORE important issues to ponder upon than mere trivialities such as these incidents.