During the campaign Trump voiced support for allowing pipelines like the DAPL and KeystoneXL to be constructed. Trump keeping is promise and working to expedite the construction of these pipelines shouldn't really come as a surprise. AS far as the DAPL is concerned, it was granted approval and construction was started under Obama. It was halted for political reasons by ordering another environmental impact study, what Trump has done is remove the order for another study since one has already been completed.
With a pedantic view than Trump could be considered violating ethics rules by using his executive authority in a way that could potentially benefit him since he had investments in the company building the pipeline. Trump's actions are defensible since his actions were only to continue an existing project that was halted for political reasons and he didn't choose who builds the pipeline or exempt them from having to follow existing regulation.