Some random causes: somewhere down the line, oil was applied to latex painting. this could cause the sheeting action The wood could have been leaching moisture or sap and not completely dried before that initial paint job. The paint was inferior. The paint flashed dried or was painted on hot surfaces or was painted on a high humidity day.
Now, sanding, I would recommend to take off all layers that give up readily and feather all other areas. Remember if it comes off easily, it will peel again. Rough up the surfaces all over with sandpaper, not to sand down to bear wood, but to cause small scratches for next paint layer to adhere.
Scrape everywhere to find loose areas. Concentrate on heavy priming on any bear wood areas, but prime the entire surface.
Here is the kicker: USE only a premium quality of paint, NOT any paint from any home center. Plan on paying $18 + a gallon. You buy the paint and have the contractor apply it. Many contractors use contractor grade paint which they can get cheap and save money. YOU DO NOT WANT THIS..
If done right and with the right tip and the right professionals, spraying on the finish coat will give you as good or better finish as hand. I would definately hand brush the primer. But, if he/she has a good rig and proper tip, then definately spray. Its more uniform, its thicker, its more effective.
FYI, tip sizes and what is the advantages and dis advantages.
Tip sizes come in three numbers. Ie .517, .413 and so on. The first number: take that number and multiply by 2 and that is your fan spread at 12 inches from the surface. the second and third number together is the orfice size.
Many contractors for speed will get a high first number so that the painting swath is wider and they will finish quicker.
Also, when spraying (I ) spray horizontally and overlap the previous swath one half. So, I am getting more paint on the surface.
Secondly, paint is advertised by 15 year, 25 year, and so on. This is pure hog wash.
Any paint will need re applying in 5 to 7 years regardless. so, if your expecting 10 years for this coating, you are really pushing it.
I repaint my home, because I love to paint, and I hate to prep work, every 4 years. usually all you have is some spot caulk, spot spackle and its a straight paint job. Its 1/2 the job.