Yes, index reorganization can cause a lot of I/O depending on how much work the reorganize decides it needs to perform to organize the index. They do not cause long-running blocking operations, i.e. they are online operations, but do cause short-lived blocking as they move through the index. It can impact I/O performance due to it reading index pages and writing entries to the transaction log as it rewrites pages. Ideally they should be done during off-peak hours to avoid impact to system users.