Does two different regulators act as different power supplies for MCU and Relays
It could be that your transformer is not capable of supplying the current needed to turn all of the relays on.  When you try to turn them on, the voltage probably drops enough for the MCU to Brown-out.

You need to look at how much current the transformer is able to supply. Also find out how much current your circuit needs to to turn all the relays on.

You could also monitor the MCU supply voltage while turning the relays on and see if the voltage drops.