This is a challenging sentence because it uses so many words that resemble verbs but that do not act as verbs. There is actually only one verb in the sentence: "follows". "Rains" functions as a noun here. (It looks like a verb, and it can be, but it is not here.) "To crop" functions as an adverb modifying the verb "follows". Infinitives have the capacity to act as nouns, adjectives, or adverbs. "Sprung", the past participle of the verb "spring", functions as an adjective to modify the noun "grass". Participles have the capacity to act as adjectives (or nouns or adverbs). Whenever you use a past participle as an adjective it has the special property of being retrospective. Present participles are not retrospective. Do you see the difference between "sprung grass" and "springing grass"? Your sentence also makes sense with the present participle: "The wildebeest follows the rains to crop the newly springing grass." Can you see the difference in meaning?