Tafsir of Al-Muddathir 74:50

( كَأَنَّهُمْ حُمُرٌ مُّسْتَنفِرَةٌ )

"As if they were startled wild donkeys." This is a circumstantial qualifier (ḥāl) for the hidden pronoun in [the preceding verse] muʿriḍīn (those turning away), expressed by way of inclusion. Ḥumur is the plural of ḥimār (donkey), and what is meant by it—as Ibn Abbas stated—is the wild donkey, because it is the proverbial example among them for aversion and intense flight.

Mustanfirah is derived from istanafara, meaning nafara (to flee/shy away), similar to ʿajiba (to wonder) and istaʿjaba (to be amazed). It is also said—and this is better—that the form istafʿala denotes intensification, as if the donkeys, due to the intensity of their running, are seeking to flee of their own accord. The meaning is: they are like donkeys that are extremely averse and skittish.