Hi Steve
All carpets when they leave the factory are acidic due to the acid dyes that are used.
Most cleaning chemicals are alkaline and if these are left in the carpet they can, over a period of time, cause problems. On certain items they can affect colours on others they can cause a breakdown of some of the fibres etc. plus they can be the cause of resoiling.
One way around this is to pre-spray the carpet with an appropriate pre-treatment solution and use an acidic rinse agent through the machine to neutralise. They can also go a long way in preventing cellulosic browning problems due to overwetting.
Hope this helps
Derek