All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
The majority of the Muslim scholars are of the view that urinating standing up when there is no sound excuse to do so is disliked since it contradicts the Sunnah of the Prophet (PBUH). Aisha (May Allah be pleased with her) denied that the Prophet (PBUH) has ever urinated standing up. This is according to the narration of Bukhari and Muslim.
Doing so is also disliked because the Awrah of the one who is urinating standing up could be seen and drops of urine could get on his body and clothes. The Messenger of Allah (PBUH) happened to pass by two graves and said: "They (their occupants) are being tormented, but they are not tormented for a grievous sin. One of them carried tales and the other did not keep himself safe from being defiled by urine." [Muslim].
Furthermore, Al-Imam An-Nawawi (May Allah have mercy on his soul) said: "Urinating standing up when there is no sound excuse to do so is disliked, but it isn`t when there is one, and this is the reliable opinion of our Madhab (Shafie' school of thought)." [Al-Majmoo', vol.2/pp.100]. Kindly refer to [Al-Mawso`a Al-Fiqhia vol.9/pp.34]. And Allah Knows Best.