They didn’t do it on purpose 
This is obviously one of the downsides to an app based bank. That being said you could still make card payments, you could still phone them and you could still access the Web version.
Ultimately it was for a few hours at most if I remember correctly. Hardly grounds for legal action and arguably not that disruptive.
FYI Lloyd’s, Halifax and the Bank of Scotland banking apps all went down the other day. It happens 