I've experienced the same thing on my Tyros. But I solved the problem after adjusting the Master EQ in the Mixing Console. I used Leo Dunne's "Medium" settings (with a slight variation on a couple settings) - from the PSR Tutorials site which can be found Here

PS: The settings are mainly used in conjunction with the Tyros' Vocal Harmonizer but it seems to have corrected my volume spike problem that I had in the process.

Also you may want to adjust your Aftertouch settings in the Function/Controller/Keyboard/Panel Menu.

I have the Initial Touch set to "Normal" and the Aftertouch to "Hard" on my Tyros.

Hope this helps.

Best regards,
Mike