MySQL sudden restart
Today I found new logs at /var/log/upstart

So, terminating with signal 1 was the after effect but not the cause.

I saw next information 

It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 5016582 K  bytes of memory
Hope that's ok; if not, decrease some variables in the equation.


So, I do assume, that the reason was in memory excess. Thus, I decreased max_threads (actually, it's the max_connections parameter) to have a security delta of memory.