The very first step in the online backup is to take a backup of the catalog tablespace
and this copy of the catalog is required to be a consistent one, For this DB2 takes table level share locks (not sure if it is S locks or some internal lock simialr to S)
When a runstats, load or reorg is running, an update lock is taken the rows of catalog tables that reference the user table on which the utilities are running.
As you can see, the above two are conflicting locks.
Therefore, if a backup starts when utilities are running, the backup waits for locktimeout seconds to obtain locks on the catalog. If it is unable to, then just like any other application, the backup fails.
Remember, that if a backup has finished copying catalog tablespace, then utilites can start on tables and both of them can contine in paralllel. In this case, just like for any other table, chnages to the catalog tables is 'tracked' using the transaction logs.
Hope this answers your question