hi,
I've seen EX4550 restarts related to SFP insertion/removal. In my case it was with third-party SFPs and 12.2R4.5.
The problem may be fixed in newer Junos code:
Issues Resolved in Release 12.2R6
EX4500 switches might reboot suddenly because they have accessed an invalid register value for a port; this problem might occur when you insert or remove SFPs, or exchange 10-gigabit and 1-gigabit SFPs in a specific port. [PR/891733: This issue has been resolved.]
jtb