If you are kind hearted and intend to give up your seat to someone, make sure that person see you and walk near to you before you stand up. If necessary (if an elderly person), hold his/her hand and guide him/her to sit safely before you move away.
Strictly speaking, when you stand up, anybody can take your seat if you don't announce your intention.
Sometimes a simple scenario can aggravate to a mindless destructive scene because of what is said and counter-said. The original goodwill is lost.