This can be due to drag produced by an atmosphere due to frequent collisions between the satellite and surrounding air molecules. The drag experienced by the object is larger in the case of increased solar activity, because it heats and expands the upper atmosphere. For larger bodies, tidal effects can cause orbital decay, and for even larger ones gravitational radiation can have an effect.
By atmospheric drag 
A major cause of orbital decay for satellites in low Earth orbit is the drag of Earth’s atmosphere. During solar maxima the Earth's atmosphere causes significant drag up to a hundred kilometers higher than during solar minima.
Atmospheric drag resulting in satellite re-entry can be described by the following sequence:
- lower altitude → increased orbital speed → denser atmosphere → increased drag → increased heat → usually burns on re-entry
Orbital decay thus involves a positive feedback effect, where the more the orbit decays, the lower its altitude drops, and the lower the altitude, the faster the decay. Decay is also particularly sensitive to external factors of the space environment such as solar activity, which are not very predictable.
Atmospheric drag exerts a significant effect at the altitudes of space stations, space shuttles and other manned Earth-orbit spacecraft, and satellites with relatively high "low earth orbits" such as the Hubble Space Telescope. Space stations typically require a regular altitude boost to counteract orbital decay (see also orbital station-keeping). Uncontrolled orbital decay brought down the Skylab space station, and (relatively) controlled orbital decay was used to de-orbit the Mir space station. Orbital boosts for the International Space Station (ISS) are regularly needed, and are one limiting factor for the length of time the ISS can go between visits from transit spacecraft.
Regular orbital boosts are also needed by the Hubble Space Telescope, though on a longer time scale, due to its much higher altitude. However, orbital decay is also a limiting factor to the length of time the Hubble can go without a maintenance rendezvous, the most recent performed successfully by STS-125, with space shuttle Atlantis launching May 11, 2009.
By tidal effects 
An orbit can also decay by tidal effects when the orbiting body is large enough to raise a significant tidal bulge on the body it is orbiting and is either in a retrograde orbit or is below the synchronous orbit. The resulting tidal interaction saps momentum from the orbiting body and transfers it to the primary's rotation, lowering the orbit's altitude until frictional effects come into play.
By gravitational radiation 
Whenever two masses orbit each other, the combined effect of the spacetime curvature of the moving objects produces gravitational waves which carry away orbital energy. For small masses this effect is negligible, but for very massive objects like black holes and neutron stars the energy carried away can be rapid enough to cause their orbits to spiral in on each other, eventually merging the two masses.