Large spike at beginning of signal after applying Bandpass filter
What you see as a spike at the beginning of the filter output is the impulse response of the bandpass filter itself. This would happen as a transient effect and it will be more pronounced if the filtered signal is sinusoidal + dc in nature. If it were a pure zero mean noise than the spike would be somewhat obscured. 

You can either use a group delay shift on the output to extract the relevant portion without that spike, or try to set the initial conditions of the filter to disable the transient reponse but almost always the first approach is preferred.