Sounds like it's crashing to me... what synth are you using? What version of iPad do you have? Does it happen every time? Can you repeat the problem? If so explain in detail so the developers can verify and fix the problem if it's there.
OS 10.10 iMac (mid 2012) i5, 16gb ram, Scarlett 18i20, Cubase 8.0.5 64bit. CMC-TP, CMC-CH, CMC-QC, CMC-AI, CMC-PD