Flailing, shaking, convulsing, weak chicken- potential causes

Hello fellow hen lovers. Today I came home to a little shock in my chicken run. Here's the sCOOP: I had 29 birds - 2 adult hens who free range and 27 2 month old pullets, who stay in the chicken run near the coop (to keep them safe from hawks/predators). None of the chickens displayed any signs of illness this afternoon. Around 7 PM, 1 was totally stiff and dead (still warm) and another was on its way. The second, dying chicken was randomly periodically flailing and flapping her wings, but couldn't stand, drink, eat, keep her eyes open. It looked like she was yawning over and over again, and she had leaves/brush stuck in her mouth that she couldnt get out.

Every other chicken I've seen in this condition has died - and I figured, especially given the other mysterious death in the flock, that this one would too. Wrong. I transported it into my garage and It has made a full / uncanny recovery within 3 hours of being on its death bed. So I have a few questions, some more information, and a plea for help to a more knowledgeable chicken owner.

Info:
-Both birds affected were Buff Orpingtons. Since receiving the birds in the mail, 6 buff orpingtons have died altogether, and only 1 of the other breeds (who make up 67%.) These are the first two deaths since they were mailed to us, though.

-The birds are very healthy looking and basically fully feathered. They have access to water and food, 24-7.

-These 2 month old chickens were indoors until Friday. It is now Sunday night. The indoor garage temperature is 55 degrees. Outdoors its low 60s in the day, low 30s at night. I figured there are 30 of them so they should be fine without a heat lamp.
-Inspecting the coop at night, some chicks are huddling together and some are sitting by themselves. None are making any noise. The coop is sealed and makeshift insulated with a tarp. Their bedding is dry.

-The run is located on the border of grass and the woods. There is ivy growing around it. There are wineberry bushes (not in bloom) and a couple other viney/ivy like plans growing on top of a wooden stump. Could this be a result of chickens eating a poisonous plant? Could an item eaten, in general, cause symptoms like this?

The chicken is now in the garage shaking slightly, somewhat lethargic, but it is able to roost, and its eyes are open and lively. It is in way better shape than it was only 3 hours ago.

What could cause flailing, shaking, convulsing, yawning (really every chicken does this when its dying,) paralysis in legs, and sudden near death with radical improvement only hours later?!?!?