Cassandra compaction tasks number keep growing - cassandra

Question

I`m using Cassandra dsc 2.1.5 with 3 nodes, and the following table description:
cqlsh> DESCRIBE TABLE mykeyspace.mytable;
CREATE TABLE mykeyspace.mytable (
a text,
b text,
c timestamp,
d timestamp,
e text,
PRIMARY KEY ((a, b), c)
) WITH CLUSTERING ORDER BY (c ASC)
AND bloom_filter_fp_chance = 0.01
AND caching = '{"keys":"ALL", "rows_per_partition":"NONE"}'
AND comment = ''
AND compaction = {'min_threshold': '4', 'class': 'org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy', 'max_threshold': '32'}
AND compression = {'sstable_compression': 'org.apache.cassandra.io.compress.LZ4Compressor'}
AND dclocal_read_repair_chance = 0.1
AND default_time_to_live = 0
AND gc_grace_seconds = 864000
AND max_index_interval = 2048
AND memtable_flush_period_in_ms = 0
AND min_index_interval = 128
AND read_repair_chance = 0.0
AND speculative_retry = '99.0PERCENTILE';
The first and the third nodes are working fine (all have the same cassandra.yaml) but the second node started to have more and more pending compaction tasks that I can see with nodetool compactionstats -H command. The situation is so bad that my spark jobs get stuck and work only when I completely shut down the second node.
I have around 130G free on the second node...
Also, here a cfstatss state:
> nodetool cfstatss mykeyspace.mytable
Keyspace: mykeyspace
Read Count: 0
Read Latency: NaN ms.
Write Count: 54316 [1/1863] Write Latency: 0.1877597945356801 ms.
Pending Flushes: 0
Table: mytable
SSTable count: 1249
Space used (live): 1125634027755
Space used (total): 1125634027755
Space used by snapshots (total): 0
Off heap memory used (total): 1202327957
SSTable Compression Ratio: 0.11699340657338655
Number of keys (estimate): 34300801
Memtable cell count: 758856
Memtable data size: 351011415
Memtable off heap memory used: 0
Memtable switch count: 10
Local read count: 0
Local read latency: NaN ms
Local write count: 54319
Local write latency: 0.188 ms
Pending flushes: 0
Bloom filter false positives: 0
Bloom filter false ratio: 0.00000
Bloom filter space used: 48230904
Bloom filter off heap memory used: 48220912
Index summary off heap memory used: 11161093
Compression metadata off heap memory used: 1142945952
Compacted partition minimum bytes: 925
Compacted partition maximum bytes: 52066354
Compacted partition mean bytes: 299014
Average live cells per slice (last five minutes): 0.0
Maximum live cells per slice (last five minutes): 0.0
Average tombstones per slice (last five minutes): 0.0
Maximum tombstones per slice (last five minutes): 0.0
What can be the problem?