Front step Victorian tiling repair

[BarumMike]

The photo shows our front steps which are probably Victorian.  The risers of the bottom three steps but not the top one have blown outwards cracking and loosening the tiles.  A number of questions.  Am I right in assuming this is due to water penetration causing expansion of the substrate below each step?  How far back should I go excavating below each step before rebuilding the riser and laying the replacement tiles?  I will probably leave the top step/riser undisturbed and just replace the two broken long yellow tiles.  Should I assume I will not be able to recover any tiles intact from the bottom three risers when ordering replacements?  Any further tips?