The first answer to your question can be answered. Basically, the cause of sagging or warpage in SH tube elements can be from either higher than normal tube metal temperatures that allows the elements to permanently deform or creep at elevated temperature, or failed assembly supports or fouled assemblies that increase bending forces on the assemblies resulting in creep of the material.

The second question has many possible answers related to combustion, steam flow, feedwater flow, fouling, etc.

A magnetite Fe₃O₄ scale often forms over the years in superheater steam tubes & boiler water tubes.  Scale thicknesses of 0.015-0.026" lead to metal temperature increases of some 25-75^oF.  This can lead to various failure modes:
1) Creep rupture itself,
2) Metallurgical transformations (carbide spheroidization, graphite formation),
3) Hot corrosion due to melting of the external ash scale.

For more details, search for articles, chapters & books by David N. French, a boiler failure investigator. E.g.,
Creep and Creep Failures (1991),
Short-Term High Temperature Failures (1991)
Fuel Ash Corrosion (1992)
Microstructural Degradation (1991)
from the
National Board Classic Series Technical Articles at http://www.nationalboard.org/NationalBoard/Articles/Classics/Classic_Articles.aspx

"Corrosion of Superheater and Reheaters in Fossil Fired Boilers", pp. 407-415, and "Microstructural Degradation of Boiler Steels," pp. 417-425, in Environmental Degradaion of Engineering Materials in Aggressive Environments (1981).

Metallurgical Failures in Fossil Fired Boilers (2nd Edn.), David N. French, 528 pages (1993). The Table of Contents & Index are on-line at
http://www.amazon.com/gp/product/0471558397/ref=sib_rdr_dp/104-1073088-9143130?%5Fencoding=UTF8&me=ATVPDKIKX0DER&no=283155&st=books&n=283155

Probably the worst culprit in causing warpage to horizontal tube assemblies and pendant sections or assemblies in boilers that I have seen is short term overheat during start-up. In many cases, the boiler metal temperatures are not monitored and during start-up excessive heat is required to boil off condensate that is trapped in tube bends. The result is over-firing to get steam flow thru the circuit and warpage.

During start-up, the operators need to monitor gas and if possible metal temperatures to properly boil out the condensate. This warpage can happen on relatively new steam circuits, or steam circuits that have been in service and in-experienced operators are brought in.

I would think that some additional information would be useful.

For example, what is the boiler type (coal, oil-fired or HRSG) and what is the orientation of the tubes (vertical or horizontal).

Also important is the method is the method of supporting the tubes and maintaining alignment.