I opened the hive yesterday to remove the Formic pro pads I treated with 2 weeks earlier and with the intent to start feeding and was sad to find an empty hive.  There were about 50 bees and a few yellow jackets working the empty, chewed out frames.  My assumption was that the majority of the robbing happened after they already absconded, but the question remains, why did they leave?  

Anyone want to speculate if the Formic pro treatment can trigger this? (entrance reducer was removed, slight crack of the cover, temperature window was obeyed).

Or was it the robbing pressure?  I just don't have a sense of how heavy that has to be to cause them to vacate.  I've been noticing a few yellow jacket-bee battles at the entrance, but no frenzy.  I had never seen any yellow jackets actually in the hive (until post absconding).

Hive was a first year from a nuc: two deeps- bottom one full and second one about 3/4 of the way full of honey.  

Thanks for any advice or opinions!  

Nicole