It's clear to me that both are related, so both have to decrease.
It is clear that allelic diversity has a huge fall because there was lost of alleles (specially the rare and uncommon ones that, despite contributing less to allelle frequencies, each one has the same weight to allelic diversity, take a look at the other graphic to understand what I'm talking about).
Now, heterozygosity is specifically related to allele frequencies, and since the most rare/uncommon don't contribute much to this, the only alleles that will have a large impact on the decrease of heterozygosity are the more/most common.
Since that after a bottleneck, (probably) the alleles remaining are the most common, heterozigosity will decrease less than allelic diversity.
I'm aiming for some complementary ideas about this, specially when it comes to statistical models that describe heterozygosity. It's hard for me to understand the relation between allele frequencies and heterozygosity when it comes to this case - in what manner do allele frequencies influence H?