Some air must have escaped past the boot clamps.
Check it over real good to make sure there isn't a pin hole in the boot.
Take your time and pull it back into it's original shape with your fingers. It'll go, just takes a few swear words and time.
I've seen this a bunch of times, never bothered to figure out why it happens. Just slide a grease needle, or any hollow needle, up the axle shaft under the small end of the boot to allow air to get into the boot.
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