Simple math, ALT/BTC ratio would decrease if:
- ALT coins value drops (i.e. people are selling the ALT coin or trading it for BTC or other ALT coins).
and/or
- BTC value increases (i.e., more demand for BTC from various accounts including USD and ALT coin conversion)
Similarly, ALT/BTC ratio would increase if:
- ALT coins value increases (i.e. people are buying an ALT coin by trading BTC or other ALT coins).
and/or
- BTC value decreases (i.e., BTC is cashed out for USD or other currency or traded for ALT coins)