How to set limits of Y-axes in countplot?
Countplot from seaborn will not work as you expect. When you calculate the frequencies, you want to plot the values in p.values as they appear. Countplot will take a dataframe where labels are not aggregated and then count each one of them, as it did in the first case.

So countplot will be appropriate for the case where your dataframe looks like:

index | reason |
0        EMS
1        EMS
2        Traffic
3        Fire
4        Fire
5        EMS
6        Traffic
...


In the second case you already have your frequencies:

index | reason |
EMS       10
Traffic   21 
Fire      15


Then count plot will just count the lines and it will be one for each, that is why your plot looks like that.

To solve your problem you could just plot using .plot from pandas:

df['reason'].value_counts(normalize=True).plot(kind='bar')


Where the parameter normalize=True will show normalized frequencies instead of raw count values.