About deadlock for drop index command on big size table

Hello,

I tried to call drop index on a big table(around 3 million records)  
through non-multi thread script, and got deadlock.

Same drop index command in the same script on a smaller size table no  
deadlock returned.

May I know that may cause the problem please?

Thanks a lot.

[Block info]
  blocking_pid | blocked_pid | blocked_query
--------------+-------------+---------------
          123 | 7298 | DROP index idx1
          123 | 7298 | DROP index idx1