[neopallium]

Either they had captured packets from Lavabit that they wanted to decrypt, or they wanted to be able to decrypt all traffic to/from Lavabit.

With how much traffic the NSA was collecting and archiving during that time, it is very likely that they had captured traffic that they thought might contain messages to/from Snowden.

[pedrocr]

Hopefully Lavabit was using TLS with forward secrecy making this impossible:

https://en.wikipedia.org/wiki/Forward_secrecy