If you have service contract with Juniper to log a ticket with JTAC, please open a case with JTAC against the platform (EX & SRX). JTAC folks will take care of the rest for you to provide an RCA.
If you do NOT have service contract with Juniper, then today is your lucky day, will tell you what to do in the following steps
Archive the /var/log/* from EX & SRX and check for core dump if any
In those var/log/ Check for messages log, chassisd log, masterships log and interactive logs files against the device reboot time stamp.
Look out for any interesting logs.. again at some point/stage, you will need Juniper TAC to help understand those logs to conclude an RCA from vendor standpoint. Or if you can analyze them by yourself to determine the cause.
If a core dump generated (if any), you will need JTAC to decode and help you find RCA.
Check of PEM/PDU failure, chassis alarms etc.. but since two of devices have rebooted (Ex & Srx) i assume, check for RACK PDU short circuit outage.
There many other factors to consider and I hope you have service contract for products you have deployed to log a case with JTAC. In case of no service contract, you can try contacting the juniper customer care them to help you log a courtesy case with JTAC if you can to convence them genuinely.
personnally, I find the command show chassis routing-engine useful for a quick look. it gives a "last Reboot reason" at the end
show chassis routing-engine Routing Engine status: Slot 0: Current state Master Temperature 26 degrees C / 78 degrees F CPU temperature 26 degrees C / 78 degrees F DRAM 1950 MB Memory utilization 24 percent 5 sec CPU utilization: User 10 percent Background 0 percent Kernel 11 percent Interrupt 2 percent Idle 76 percent Model RE-EX2300C-12T Serial ID BUILTIN Start time 2019-08-29 11:17:26 EDT Uptime 35 days, 23 hours, 18 minutes, 46 seconds Last reboot reason Router rebooted after a normal shutdown. Load averages: 1 minute 5 minute 15 minute 0.62 0.52 0.42