Answer:
They all have a reading that is the same backwards.
石 SEKI, SHAKU, ishi - stone
写 SHA, utsu(ru), utsu(su) - copy, duplicate
充 JUU, a(teru) - fill, allot, apply to
泳 EI, oyo(gu) - swim
恐 KYOU, oso(reru), oso(roshii) - fear, be afraid of, frightful