Why do people wash their hands after a funeral?

Thank you for your question, The reason for washing hands after a funeral is that there are bad spirits (שדים) that can negatively affect someone who enters into a cemetery or who participates in the funeral (Magen Avraham, 4:20, Mishna Berura 4:42). Someone who didn't go into the actual cemetery but just participated in the funeral, only needs to do netilat yadaim (hand washing) if he was within four amot (approximately 2 meters) of the dead person (see Pri Megadim Eshel Avraham 4:21 ,and Kaf Hachaim 4:77). However, according to the Chazon Ish, anyone who participated in the funeral needs to do netilat yadaim (hand washing) , even if he was not within four amot of the dead (dinim vehanhagot, yoreh deah 4; 25). All the best