Spontaneously deleting records from MongoDB shards?

Question

MongoDB

I have MongoDB 3 installed.
Created within one server 4 shards, 1 config server.
Records are being added to the collection, approximately 100 records per second. There are about 100 million entries in the collection. There are no stresses. A random sharding key is selected, I add a random hash to each entry. It used to be 10 random characters, then 16. I
began to notice that after I complete the addition, after some time, records are deleted from the shards. They are deleted at approximately the same rate of 50-100 records per second. I thought it was because of the distribution that the records are being moved, but after running queries for the number of records in each shard, I came to the conclusion that about 100 thousand records are being deleted per night.
Help solve the problem.

Answer 1

db.myCollectioName.getShardDistribution();
I do not understand why a day later the reverse process begins. I tried to start adding minutes for 15, then complete and wait for the removal, but it does not happen. Everything seems to work normally.
In what direction to study?