Observe many copy threads blocking there when use pt-online-schema-change

The other day, i use pt-online-schema-change to add a column and index to a table,the table is about 1 Minion rows and the .ibd file size is about 5G Bytes.

I tried several times each time i got an error says the process is aborted duo to the critical load is reached(which is threads_running=60, i always use this and never failed before)

I found something strange using show full process list: there are so many copy threads there like:
REPLACE INTO db._some_table_new
running for a long time(tens and hundreds of seconds) and in a status of ‘update’.

As i know the pt-online-schema-change uses only one copy threads and i use this tool many times and each time i can only observe one thread chunking and copying but this time everything changed, there were so many copy threads blocking there.

I searched the documents in percona.com and can not find any information about the option about multi-threaded copying.

The table i altered this time does be accessed very frequently, but i think this will not cause a problem as long as there is a single copying thread.

Could anyone give me some help,thanks!