Intermittent loss of power to a snapped piston?What happened

It all started 3 months ago when my 1998 ford escort lx 2.0 with about 125000 miles ( but always maintained) started having a loss of power while driving, no pattern to it, I would be driving along and suddenly just slow down or be at a light and start to accelerate and just be creeping along for a bout a minute then power returns(sometimes it would sputter when when I first started it but never stalled out). I took it to the shop where I normally go for regular maintenance and it did not repeat the problem for them, so I continued driving. It would only do it every know and again until 3 weeks ago it did it 3 days in a row so I took it back to the shop and asked them to keep it for the week to see if it would do it for them, during this time it did it one time, they replaced the spark plugs/ wires as well as cleaning my fuel injectors. The day I call to see if my car is ready the said that upon test driving it after the repairs it made a loud clanking noise( not doing this before)and discovered piston pieces in my oilpan. They say due to a slipped timing belt and now I need a new engine! How could this have happened timing belt is 6 months old and replaced by them!