2 sentences

1. 你平白无故发哪门子火？

   nǐ píng​bái​wú​gù fā nǎ mén​zi huǒ?

   What are you getting so angry about for no reason?

   Source: Dong Chinese Dictionary

2. 别傻了，他怎么可能平白无故地送你这么昂贵的礼物？

   bié shǎ le, tā zěn​me kě​néng píng​bái​wú​gù de sòng nǐ zhè​me áng​guì de lǐ​wù?

   Don't be silly; why would he give you such an expensive gift for no reason at all?

   Source: Dong Chinese Dictionary