It looks as if you did not plot against the time (i.e. X-data). If you only supply the Y-data then MATLAB will simply plot the Y-data against their indices.
A similar effect would also occur if you use the same X-data for both of the Y-data, even though they were sampled at different times.
Basically there are multiple reasons why this might happen, and we don't know which one applies to your situation because you have not uploaded any code for us to review.
