What are the possible reasons a plane would do this?

There are some speed limits for certain types of airspace. Airliners are always in contact with air traffic control and in order to keep the required separation of aircraft sometimes the controllers will ask the pilot to maintain an airspeed. More than likely you experienced a slowdown while your plane was beginning an approach, During approach controllers have to maintain specific spacing between aircraft and often must slow them down behind slower aircraft. Your plane very likely was slowing down from approx 570 knots to 250 knots or less for the approach.

You're stuck with a water landing (ditching). Hopefully at altitude the pilot can see a ship and make the landing in the vicinity of the ship for quick rescue. It so rare for a plane to lose power to all engines though that you should never hear of such an incident. Depending on the route, however, Bermuda may be within gliding distance.

If you are flying level you climb. If it's a propeller plane and you don't add rudder to counter "P factor" you may also turn a bit.

its all about relative motion and being part of the earth atmosphere system, the plane is matched in speed with the earth. its the same reason we dont fly away when we jump up.

Actually, there are. Tracor Aerospace (now part of BAE) has a contract to turn them into target drones (called QF-4's), so the numbers are dwindling. The US military officially announce the decommissioning of the F4's (although actually doing so is still a work in progress, and some may still be serviceable). But the rest are unmanned drones.

There are some foreign countries still flying them, as far as I know. Parts are getting hard to find (especially engine "hot section" parts). Unfortunately, the US deliberately destroys those parts, so they cannot be used to keep other planes flying. Some of those planes still find a home, however, as "gate guards" it airports around the country.

Your question is a good one - but the answer is much more complicated that you would expect. Think about driving your car from point A to point B across a city. Lots of paths - some shorter than others, but the shortest path may not be the quickest. Or the quickest may involve a toll road - and you may or may not be in a hurry.

The usual most important factor (for commercial operations, at least) is to save money, while still arriving on time. Airplanes in the air are subject to the winds aloft, which will generally be at different strengths AND DIRECTIONS at different altitudes. Most airplanes operate more efficiently at higher altitudes (up to limits), but at those higher altitudes the plane may face stiffer headwind. Further, it costs time and fuel to climb to those altitudes, and you will not regain coming down as much as it took going up. [Think of a bicycle on hilly terrain vs. level ground.]

So what's the answer? Well, for most trips the pilot will consider all these factors. They are taught during training how to plan the flight in terms of time and fuel required, and to include in that especially the winds at different altitudes. Then they will pick the altitude, whatever that is, that maximizes the results that they consider most important.

I suspect that the idea of max climb plus glide is not a new one, and that, if it really were cost-effective on fuel, we would know about it.

Students and certificated pilots both are required to remain current and to have a check ride (flight review with an instructor) every 24 calendar months. The ones you're seeing are probably going out to the practice area to practice their flight maneuvers. If a pilot doesn't practice regularly his or her skills can deteriorate rapidly.