There was a suprising number of planes in the air around Malaysia at 1
am local time.

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A pilot flying another plane who tried to contact the pilots in the
cockpit of the Malaysia Airlines plane said he heard mumbled voices
before contact was lost.
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So another theory: Something went wrong with the cabin air inside the
plane. Either it lost pressurization (slowly) or recirculation wasn't
working - causing buidup of carbon monoxide.

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On October 25, 1999, a chartered Learjet 35 was scheduled to fly from
Orlando, Florida to Dallas, Texas. Early in the flight the aircraft,
which was cruising at altitude on autopilot, quickly lost cabin
pressure. All on board were incapacitated due to hypoxia — a lack of
oxygen. The aircraft failed to make the westward turn toward Dallas over
north Florida. It continued flying over the southern and midwestern
United States for almost four hours and 1,500 miles (2,400 km). The
plane ran out of fuel and crashed into a field near Aberdeen, South
Dakota after an uncontrolled descent. The four passengers on board were
golf star Payne Stewart, his agents, Van Ardan and Robert Fraley, and
Bruce Borland, a highly regarded golf architect with the Jack Nicklaus
golf course design company.

The NTSB determined that:

The probable cause of this accident was incapacitation of the
flight crew members as a result of their failure to receive
supplemental oxygen following a loss of cabin pressurization,
for undetermined reasons.

A possible explanation for the failure of the pilots to receive
emergency oxygen is that their ability to think and act decisively was
impaired because of hypoxia before they could don their oxygen masks. No
definitive evidence exists that indicates the rate at which the accident
flight lost its cabin pressure; therefore, the Safety Board evaluated
conditions of both rapid and gradual depressurization.

If there had been a breach in the fuselage (even a small one that could
not be visually detected by the in-flight observers) or a seal failure,
the cabin could have depressurized gradually, rapidly, or even
explosively. Research has shown that a period of as little as 8 seconds
without supplemental oxygen following rapid depressurization to about
30,000 feet (9,100 m) may cause a drop in oxygen saturation that can
significantly impair cognitive functioning and increase the amount of
time required to complete complex tasks.
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So either MH370 depressurized quickly - or slowly.

The pilots might have been able to put their masks on - or realize they
needed to put their masks on.

Perhaps they did - but their supplemental oxygen supply didn't work.

Perhaps in their confused state, with or without functional masks, they
started an emergency descent before they blacked out, causing the plane
to smash into the ocean with the pilots incapacitated on the way down.

Note also that in the flight of the Lear Jet in 1999 that the pilots,
even if they did don their masks, made no attempt at radio contact.

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2005 Helios Airways Flight 522 crash

On August 14, 2005, a Helios Airways Boeing 737-300 crashed 40 km (25
mi) from Athens after running out of fuel. An investigation later
concluded that an improper pressurization setting in the cockpit had
caused the cabin pressure to drop, and resulted in the incapacitation of
the passengers and crew. It was later determined that one of the flight
attendants had used the bottled oxygen supply and his pilot's training
to attempt to bring the plane down to a lower altitude. There were no
survivors.
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I think this is looking more and more likely - that the pilots suffered
a slow asphyxia or hypoxia, caused either by a fault in the airframe or
the misapplication or failure of some valve or switch. The plane
descended rapidly, either as the last semi-conscious act of the pilot(s)
or because of a complete lack of pilot input to the controls.

This would be expected during the early phase of the flight, as it
climbs to cruise altitude.