Exit 125 - We know the cause but why?

Past the halfway mark of this expressway, near Exit 125, our convoy slightly slowed down before we heard Artem's voice in the radio:

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*bzzt*

[Nyebo— Deadheads ahead, three in total, proceeding to slow down. We'll take care of them in just a minute or so.]

*bzzt*

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*bzzt*

"Where did it come from?"

*bzzt*

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*bzzt*

[My bet's the exit but we'll check, no need for you guys to get down.]

*bzzt*

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Then Jared joined in:

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*bzzt*

[Bro, I think they climbed the wall.]

*bzzt*

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*bzzt*

"How?"

*bzzt*

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Then a few more voices joined in:

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*bzzt*

[Yeah, how?]

*bzzt*

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*bzzt*

[Explain.]

*bzzt*

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*bzzt*