When you create a snapshot on a VM this creates a Delta Disk and the operating system writes to this file instead of the original VMDK. This file is called VM_Name-Delta.VMDK but if the system needs to refers to a file before the snapshot it will refers to VM_Name.VMDK increasing the I/O of this operation. If you take multiple snapshots you are referring to the last delta file of the last snapshot not the original VMDK thus increasing I/O.
Example.
OS ---> Snapshot (File A Created) ---> (Snapshot File B Created)
If I need to refer to File A it will be looking through 3 VMDK's to find this.
Also if you include the memory state of the VM at the time of snapshotting this creates a this again is a delta file and refers to the original memory files if needed.
A file is created this lists all the files created at the time of the snapshot process