When you start the tablet servers, kudu needs to read all of the block containers located in that tablet server.
Depending on how much data is stored there, is the amount of time it can take to open all the containers. Several variables are also involved such as disk performance, how busy is the host doing other things, if the disks are shared, etc...
If it happens to only one host, chances are this host is either facing some hardware issues or have a lot of data to process, otherwise you only have a lot of block containers to work with.
Please let us know if there are further questions we can clarify for you.