See that parasitic body diode in the MOSFET symbol? That means MOSFETs can only block current in one direction. Having them back-to-back lets it block current in both directions (i.e. AC). I'm not entirely sure why bidirectional blocking is required in this case, but is probably because whatever is supplying the DC-DC converter does not like having the DC-DC converter push current back into it when it is powering down.
You could connect it drain-to-drain if you wanted and it would still work block in both directions but it makes your gate drive much more complicated. Since the voltage that controls the MOSFETs is the voltage difference between gate and source. Tying the sources together lets you only require one gate drive circuit for both MOSFETs. If you tied drains together, each MOSFET would require a separate gate driver since the reference voltage that the MOSFET gate voltage cares about (i.e. the voltage at the source pin) is different between teh two MOSFETs.