Hi, I currently see one engine in the output of the checkengine.sh sc

There can be quite a few reason for checkpoint recommendation. For e.g. there might have been lots of transaction having gone through that engine (maybe a process invoking many sub-processes synchronously) etc., Or transaction size > certain size etc., I suggest you run checkpoint command manually

Is the transaction size influenced by the size of the CDT process variables? If that's the case that could be a reason for my case I guess. I already did the manual checkpointing. I am just curious what the causes of this could have been.

Large CDT's in high volume might possibly mean a substantial growth in KDB size. This sudden growth can also recommend checkpoint. See
forum.appian.com/.../Troubleshooting_with_Diagnostic_Scripts