a lot of session wait on lock relation

Hi,
   PGv 15.10,  many session coming in doing "insert into tablexxx
values...." in parallel, this is a range partition tables, total 12
partitions plus one default partition, only three btree indexes, no others
constraints ,no foreign key. but we see hundreds of session waiting on
"lock:relation", why inserts into partition table cause "relation lock" ?

Thanks,

James