From fence crossings and this can occur at all times of the year. Research on migrations has shown that some individuals will have up to and over 1,000 fences crossing in a migrational trip. Trail cameras from known fence crossings show them losing tufts of hair at all times of the year and some pronghorn have almost a reverse mohawk where all the hair is gone along the spine.
Also, like others have mentioned I have seen dam near an entire side of a pronghorn free of hair from it being drug across a field. Their ability to lose hair is a prime reason why taxidermist love to have an extra cape or two lying around every year as most naive people pull off good chunks while in the field.