Any reason why I would go all the way down to 60LP after losing promos?

I legit went from 0 LP to 100 LP in a single day of just constant winning, then I get into promos and suddenly everyone on my team is shit. I lose promos and go down to 60LP, then I get back into promos again and BOOM everyone on my team is shit again. Lose promos for the second time and AGAIN I'm back to 60LP. What's up with that?
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