The verse (Yirmeyahu 22:19) curses Yehoyakim saying that he will be buried "a donkey's burial". This meant that he would not be buried at all, but thrown in a garbage heap outside the city like a dead animal (Targum, Rashi and Radak), as is clear from the Gemara in Sanhedrin 82a (see Rashi there). According to one Midrash (Vayikra Rabah 19:6), Yehoyakim's flesh was fed to the dogs, like that of a dead donkey.

In Bava Basra 101b, we are told that Rebbi Yochanan called burial in a standing position "a donkey's burial." Rashbam (and Yad Ramah, in more detail) explains that one does not dig a grave for dead donkeys. Rather, they are thrown into a deep ditch or gulley. These are likely to be narrow, and thus the donkey might very well end up wedged in the ditch in a standing position.

Best wishes,

Mordecai Kornfeld