Secondary keeps rolling back - mongodb

Question

In last 7 days, three times our secondary servers went down with the following message. What these errors mean? Why does it rollback? I have attached the screen shot of the oplog window and replication lag.
Around 4AM the server went down. Around 3:50 the replication lag went to 300 seconds, but that is just 5 mins, the node has more oplog window.
We take backups using MMS from one of the secondary, does this could be the cause of issue?
Mon May 19 03:50:27.146 [rsBackgroundSync] replSet syncing to: xxxx.prod.xxxx.net:17017
Mon May 19 03:50:27.231 [rsBackgroundSync] replSet our last op time fetched: May
19 03:50:16:152
Mon May 19 03:50:27.231 [rsBackgroundSync] replset source's GTE: May 19 03:50:16
:153
Mon May 19 03:50:27.231 [rsBackgroundSync] replSet rollback 0
Mon May 19 03:50:27.231 [rsBackgroundSync] replSet ROLLBACK
Mon May 19 03:50:27.231 [rsBackgroundSync] replSet rollback 1
Mon May 19 03:50:27.231 [rsBackgroundSync] replSet rollback 2 FindCommonPoint
Mon May 19 03:50:27.232 [rsBackgroundSync] replSet info rollback our last optime
: May 19 03:50:16:152
Mon May 19 03:50:27.232 [rsBackgroundSync] replSet info rollback their last opti
me: May 19 03:50:16:155
Mon May 19 03:50:27.232 [rsBackgroundSync] replSet info rollback diff in end of
log times: 0 seconds
Mon May 19 03:50:27.691 [rsBackgroundSync] replSet rollback found matching event
s at Mar 13 06:12:22:11
Mon May 19 03:50:27.691 [rsBackgroundSync] replSet rollback findcommonpoint scan
ned : 222891
Mon May 19 03:50:27.691 [rsBackgroundSync] replSet replSet rollback 3 fixup
Mon May 19 03:50:30.065 [rsBackgroundSync] replSet rollback 3.5
Mon May 19 03:50:30.065 [rsBackgroundSync] replSet rollback 4 n:7018
Mon May 19 03:50:30.065 [rsBackgroundSync] replSet minvalid=May 19 03:50:16 5379
e1e8:155
Mon May 19 03:50:30.065 [rsBackgroundSync] replSet rollback 4.6
Mon May 19 03:50:30.065 [rsBackgroundSync] replSet rollback 4.7
Mon May 19 03:50:30.443 [rsBackgroundSync] ERROR: rollback cannot find object by
id
Mon May 19 03:50:30.444 [rsBackgroundSync] ERROR: rollback cannot find object by
id
Mon May 19 03:50:30.444 [rsBackgroundSync] replSet rollback 5 d:4 u:7016
Mon May 19 03:50:30.460 [rsBackgroundSync] replSet rollback 6

Answer 1

We found oplog in the primary got corrupted somehow. We found it by running hte following queries
db.oplog.rs.find().sort({$natural:1}).explain()
db.oplog.rs.find().sort({$natural:-1}).explain()
So we did a primary step down, and did a fresh sync.