Android paid VPN connection keeps dropping without warning

Regardless of what device I use, which VPN provider I use (PIA, CyberghostVPN and many others), which server I use and which protocols I use, my connection will eventually drop without warning and I will unknowingly be accessing the internet UNPROTECTED at my favorite crowded hotspot!!!

My questions are:

1. Why is this happening?
2. What can be done about it?