From the Internet > An idiom
Put somebody off their stride> especially British English.
Knock/throw/keep somebody off stride > American English.
= to make someone unable to do something effectively, by not allowing them to give all their attention to it / to interfere with one's normal and natural progress or rate of progress.
Your startling comments put Larry off his stride for a moment/ He was put off his stride by an interruption from the audience/ His testimony threw the defense off stride.
The unexpected has interfered with his natural tendency to plan and keep things as planned .