Why the voltage suddenly reduces when thyristors change from forward blocking to conduction mode
An SCR is not the same as a diode, the foldback indicates that there is voltage that the SCR will start conducting, without specifically applying gate current (which is normally the way you would use it).  It in effect an avalanche affect.

For a nice explanation (sorry for the fragmented link on the original, google seems to have it right):

https://www.google.com/url?sa=t&amp;rct=j&amp;q=&amp;esrc=s&amp;source=web&amp;cd=2&amp;ved=2ahUKEwjS0-aLh5DeAhVpw1kKHYlLD-QQFjABegQIBBAC&amp;url=https%3A%2F%2Fcie-wc.edu%2FSCRs9_29_11.pdf&amp;usg=AOvVaw1fnW528a8o0eYbvpII_Cfr