My 19 month old son was a perfect sleeper... Until 3 months ago. Suddenly every night or 2nd night would wake at 11pm or so screaming for at least 2 hours. Would not accept a cuddle would stand in his cot screaming holding his arms up, would carry on in the lounge, even in our bed & would finally pass out from exhaustion 2 hours later. I had also just become pregnant with #3 which his crazy antics coincided with.
Anyway suddenly 2 weeks ago he is back to "normal"... I did nothing different he just took his dummy peacefully and since then sleeps peacefully once again.

So I have no answer for you but might just be a crazy stage with no reason like my son. Now DS1 is becoming the crazy one!