If a speaker is rated for 50 watts RMS, and the amp is rated at 25 watts. The speaker is not going to force extra wattage out of the amp...harming it. The signal flows from the headunit to the amp to the speakers.

You said this happen after you installed the speakers. An easy way to try and solve this is to deduct all the variables. Take those speakers out and plug in your old ones.

This is all assuming that you installed everything neatly and correctly.