[steve86]

A couple possibilities for the suppression of Syrian air defenses:

1) Cutting edge U.S./Israeli programs in remote digital signal injection, i.e. remote hacking of enemy control systems and changing code or simply "turning off".
2) A more political possibility: Putin was given some sort of ultimatum through back channels if he didn't shut off the systems. So through some backdoor, Russian technicians shut down the defenses.

I don't believe the air defenses were destroyed by bomb attacks, as this would have been observed and reported.

[steve86]

Also meant to add 3) Israeli Special Forces sabotaged the air defenses.