Many variables in S1914, the 9 inf units even at 1/3rd strength while at sea have 3 hit points and your cruisers depending on condition only have 2 hit points each... also the inf units would get a morale boost after defeating first cruiser... unless you sat and observed it all play out, no one will now be able to tell you why...
Speculation can say, there could have been an invisible opponent Sub with 4 hit points in middle of the fight, or maybe there were planes or a rail gun in the equation if this skirmish took place near a coast... also opponent might have boosted inf morale with GOLD just prior to fighting your cruisers or in between killing the 1st and 2nd cruiser.
Again, many variables that your scenario does not describe, and we have no screen shot to look at... when fighting I can tell you that in my experience HIT POINTS are most important thing to monitor. You can click on any unit and the (i) info about that unit to decipher the hit points.
In Call of War (CoW) when the went to the similar design changes, there called CoW 1.5 there was a phase where convoys were invincible and were taking down substantial fleets ! Probably the same algorithmic bug.