Pulldown resistor on MCU output?
Upon reset a microcontroller always sets all its I/O to input, so that's high impedance. The reason is that this way you can force a known level by pulling up or down, even before the microcontroller has initialized the I/O, so that it can't unintentionally activate some external device, like your SSR.  

Apparently it doesn't work as intended in your situation. I call software bug in initialization routine. Make sure you set the output low before setting the I/O pin to output.